1======================== 2ftrace - Function Tracer 3======================== 4 5Copyright 2008 Red Hat Inc. 6 7:Author: Steven Rostedt <srostedt@redhat.com> 8:License: The GNU Free Documentation License, Version 1.2 9 (dual licensed under the GPL v2) 10:Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, 11 John Kacur, and David Teigland. 12 13- Written for: 2.6.28-rc2 14- Updated for: 3.10 15- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt 16- Converted to rst format - Changbin Du <changbin.du@intel.com> 17 18Introduction 19------------ 20 21Ftrace is an internal tracer designed to help out developers and 22designers of systems to find what is going on inside the kernel. 23It can be used for debugging or analyzing latencies and 24performance issues that take place outside of user-space. 25 26Although ftrace is typically considered the function tracer, it 27is really a framework of several assorted tracing utilities. 28There's latency tracing to examine what occurs between interrupts 29disabled and enabled, as well as for preemption and from a time 30a task is woken to the task is actually scheduled in. 31 32One of the most common uses of ftrace is the event tracing. 33Throughout the kernel is hundreds of static event points that 34can be enabled via the tracefs file system to see what is 35going on in certain parts of the kernel. 36 37See events.txt for more information. 38 39 40Implementation Details 41---------------------- 42 43See :doc:`ftrace-design` for details for arch porters and such. 44 45 46The File System 47--------------- 48 49Ftrace uses the tracefs file system to hold the control files as 50well as the files to display output. 51 52When tracefs is configured into the kernel (which selecting any ftrace 53option will do) the directory /sys/kernel/tracing will be created. To mount 54this directory, you can add to your /etc/fstab file:: 55 56 tracefs /sys/kernel/tracing tracefs defaults 0 0 57 58Or you can mount it at run time with:: 59 60 mount -t tracefs nodev /sys/kernel/tracing 61 62For quicker access to that directory you may want to make a soft link to 63it:: 64 65 ln -s /sys/kernel/tracing /tracing 66 67.. attention:: 68 69 Before 4.1, all ftrace tracing control files were within the debugfs 70 file system, which is typically located at /sys/kernel/debug/tracing. 71 For backward compatibility, when mounting the debugfs file system, 72 the tracefs file system will be automatically mounted at: 73 74 /sys/kernel/debug/tracing 75 76 All files located in the tracefs file system will be located in that 77 debugfs file system directory as well. 78 79.. attention:: 80 81 Any selected ftrace option will also create the tracefs file system. 82 The rest of the document will assume that you are in the ftrace directory 83 (cd /sys/kernel/tracing) and will only concentrate on the files within that 84 directory and not distract from the content with the extended 85 "/sys/kernel/tracing" path name. 86 87That's it! (assuming that you have ftrace configured into your kernel) 88 89After mounting tracefs you will have access to the control and output files 90of ftrace. Here is a list of some of the key files: 91 92 93 Note: all time values are in microseconds. 94 95 current_tracer: 96 97 This is used to set or display the current tracer 98 that is configured. 99 100 available_tracers: 101 102 This holds the different types of tracers that 103 have been compiled into the kernel. The 104 tracers listed here can be configured by 105 echoing their name into current_tracer. 106 107 tracing_on: 108 109 This sets or displays whether writing to the trace 110 ring buffer is enabled. Echo 0 into this file to disable 111 the tracer or 1 to enable it. Note, this only disables 112 writing to the ring buffer, the tracing overhead may 113 still be occurring. 114 115 The kernel function tracing_off() can be used within the 116 kernel to disable writing to the ring buffer, which will 117 set this file to "0". User space can re-enable tracing by 118 echoing "1" into the file. 119 120 Note, the function and event trigger "traceoff" will also 121 set this file to zero and stop tracing. Which can also 122 be re-enabled by user space using this file. 123 124 trace: 125 126 This file holds the output of the trace in a human 127 readable format (described below). Note, tracing is temporarily 128 disabled while this file is being read (opened). 129 130 trace_pipe: 131 132 The output is the same as the "trace" file but this 133 file is meant to be streamed with live tracing. 134 Reads from this file will block until new data is 135 retrieved. Unlike the "trace" file, this file is a 136 consumer. This means reading from this file causes 137 sequential reads to display more current data. Once 138 data is read from this file, it is consumed, and 139 will not be read again with a sequential read. The 140 "trace" file is static, and if the tracer is not 141 adding more data, it will display the same 142 information every time it is read. This file will not 143 disable tracing while being read. 144 145 trace_options: 146 147 This file lets the user control the amount of data 148 that is displayed in one of the above output 149 files. Options also exist to modify how a tracer 150 or events work (stack traces, timestamps, etc). 151 152 options: 153 154 This is a directory that has a file for every available 155 trace option (also in trace_options). Options may also be set 156 or cleared by writing a "1" or "0" respectively into the 157 corresponding file with the option name. 158 159 tracing_max_latency: 160 161 Some of the tracers record the max latency. 162 For example, the maximum time that interrupts are disabled. 163 The maximum time is saved in this file. The max trace will also be 164 stored, and displayed by "trace". A new max trace will only be 165 recorded if the latency is greater than the value in this file 166 (in microseconds). 167 168 By echoing in a time into this file, no latency will be recorded 169 unless it is greater than the time in this file. 170 171 tracing_thresh: 172 173 Some latency tracers will record a trace whenever the 174 latency is greater than the number in this file. 175 Only active when the file contains a number greater than 0. 176 (in microseconds) 177 178 buffer_size_kb: 179 180 This sets or displays the number of kilobytes each CPU 181 buffer holds. By default, the trace buffers are the same size 182 for each CPU. The displayed number is the size of the 183 CPU buffer and not total size of all buffers. The 184 trace buffers are allocated in pages (blocks of memory 185 that the kernel uses for allocation, usually 4 KB in size). 186 If the last page allocated has room for more bytes 187 than requested, the rest of the page will be used, 188 making the actual allocation bigger than requested or shown. 189 ( Note, the size may not be a multiple of the page size 190 due to buffer management meta-data. ) 191 192 Buffer sizes for individual CPUs may vary 193 (see "per_cpu/cpu0/buffer_size_kb" below), and if they do 194 this file will show "X". 195 196 buffer_total_size_kb: 197 198 This displays the total combined size of all the trace buffers. 199 200 free_buffer: 201 202 If a process is performing tracing, and the ring buffer should be 203 shrunk "freed" when the process is finished, even if it were to be 204 killed by a signal, this file can be used for that purpose. On close 205 of this file, the ring buffer will be resized to its minimum size. 206 Having a process that is tracing also open this file, when the process 207 exits its file descriptor for this file will be closed, and in doing so, 208 the ring buffer will be "freed". 209 210 It may also stop tracing if disable_on_free option is set. 211 212 tracing_cpumask: 213 214 This is a mask that lets the user only trace on specified CPUs. 215 The format is a hex string representing the CPUs. 216 217 set_ftrace_filter: 218 219 When dynamic ftrace is configured in (see the 220 section below "dynamic ftrace"), the code is dynamically 221 modified (code text rewrite) to disable calling of the 222 function profiler (mcount). This lets tracing be configured 223 in with practically no overhead in performance. This also 224 has a side effect of enabling or disabling specific functions 225 to be traced. Echoing names of functions into this file 226 will limit the trace to only those functions. 227 This influences the tracers "function" and "function_graph" 228 and thus also function profiling (see "function_profile_enabled"). 229 230 The functions listed in "available_filter_functions" are what 231 can be written into this file. 232 233 This interface also allows for commands to be used. See the 234 "Filter commands" section for more details. 235 236 set_ftrace_notrace: 237 238 This has an effect opposite to that of 239 set_ftrace_filter. Any function that is added here will not 240 be traced. If a function exists in both set_ftrace_filter 241 and set_ftrace_notrace, the function will _not_ be traced. 242 243 set_ftrace_pid: 244 245 Have the function tracer only trace the threads whose PID are 246 listed in this file. 247 248 If the "function-fork" option is set, then when a task whose 249 PID is listed in this file forks, the child's PID will 250 automatically be added to this file, and the child will be 251 traced by the function tracer as well. This option will also 252 cause PIDs of tasks that exit to be removed from the file. 253 254 set_event_pid: 255 256 Have the events only trace a task with a PID listed in this file. 257 Note, sched_switch and sched_wake_up will also trace events 258 listed in this file. 259 260 To have the PIDs of children of tasks with their PID in this file 261 added on fork, enable the "event-fork" option. That option will also 262 cause the PIDs of tasks to be removed from this file when the task 263 exits. 264 265 set_graph_function: 266 267 Functions listed in this file will cause the function graph 268 tracer to only trace these functions and the functions that 269 they call. (See the section "dynamic ftrace" for more details). 270 Note, set_ftrace_filter and set_ftrace_notrace still affects 271 what functions are being traced. 272 273 set_graph_notrace: 274 275 Similar to set_graph_function, but will disable function graph 276 tracing when the function is hit until it exits the function. 277 This makes it possible to ignore tracing functions that are called 278 by a specific function. 279 280 available_filter_functions: 281 282 This lists the functions that ftrace has processed and can trace. 283 These are the function names that you can pass to 284 "set_ftrace_filter", "set_ftrace_notrace", 285 "set_graph_function", or "set_graph_notrace". 286 (See the section "dynamic ftrace" below for more details.) 287 288 dyn_ftrace_total_info: 289 290 This file is for debugging purposes. The number of functions that 291 have been converted to nops and are available to be traced. 292 293 enabled_functions: 294 295 This file is more for debugging ftrace, but can also be useful 296 in seeing if any function has a callback attached to it. 297 Not only does the trace infrastructure use ftrace function 298 trace utility, but other subsystems might too. This file 299 displays all functions that have a callback attached to them 300 as well as the number of callbacks that have been attached. 301 Note, a callback may also call multiple functions which will 302 not be listed in this count. 303 304 If the callback registered to be traced by a function with 305 the "save regs" attribute (thus even more overhead), a 'R' 306 will be displayed on the same line as the function that 307 is returning registers. 308 309 If the callback registered to be traced by a function with 310 the "ip modify" attribute (thus the regs->ip can be changed), 311 an 'I' will be displayed on the same line as the function that 312 can be overridden. 313 314 If the architecture supports it, it will also show what callback 315 is being directly called by the function. If the count is greater 316 than 1 it most likely will be ftrace_ops_list_func(). 317 318 If the callback of the function jumps to a trampoline that is 319 specific to a the callback and not the standard trampoline, 320 its address will be printed as well as the function that the 321 trampoline calls. 322 323 function_profile_enabled: 324 325 When set it will enable all functions with either the function 326 tracer, or if configured, the function graph tracer. It will 327 keep a histogram of the number of functions that were called 328 and if the function graph tracer was configured, it will also keep 329 track of the time spent in those functions. The histogram 330 content can be displayed in the files: 331 332 trace_stat/function<cpu> ( function0, function1, etc). 333 334 trace_stat: 335 336 A directory that holds different tracing stats. 337 338 kprobe_events: 339 340 Enable dynamic trace points. See kprobetrace.txt. 341 342 kprobe_profile: 343 344 Dynamic trace points stats. See kprobetrace.txt. 345 346 max_graph_depth: 347 348 Used with the function graph tracer. This is the max depth 349 it will trace into a function. Setting this to a value of 350 one will show only the first kernel function that is called 351 from user space. 352 353 printk_formats: 354 355 This is for tools that read the raw format files. If an event in 356 the ring buffer references a string, only a pointer to the string 357 is recorded into the buffer and not the string itself. This prevents 358 tools from knowing what that string was. This file displays the string 359 and address for the string allowing tools to map the pointers to what 360 the strings were. 361 362 saved_cmdlines: 363 364 Only the pid of the task is recorded in a trace event unless 365 the event specifically saves the task comm as well. Ftrace 366 makes a cache of pid mappings to comms to try to display 367 comms for events. If a pid for a comm is not listed, then 368 "<...>" is displayed in the output. 369 370 If the option "record-cmd" is set to "0", then comms of tasks 371 will not be saved during recording. By default, it is enabled. 372 373 saved_cmdlines_size: 374 375 By default, 128 comms are saved (see "saved_cmdlines" above). To 376 increase or decrease the amount of comms that are cached, echo 377 in a the number of comms to cache, into this file. 378 379 saved_tgids: 380 381 If the option "record-tgid" is set, on each scheduling context switch 382 the Task Group ID of a task is saved in a table mapping the PID of 383 the thread to its TGID. By default, the "record-tgid" option is 384 disabled. 385 386 snapshot: 387 388 This displays the "snapshot" buffer and also lets the user 389 take a snapshot of the current running trace. 390 See the "Snapshot" section below for more details. 391 392 stack_max_size: 393 394 When the stack tracer is activated, this will display the 395 maximum stack size it has encountered. 396 See the "Stack Trace" section below. 397 398 stack_trace: 399 400 This displays the stack back trace of the largest stack 401 that was encountered when the stack tracer is activated. 402 See the "Stack Trace" section below. 403 404 stack_trace_filter: 405 406 This is similar to "set_ftrace_filter" but it limits what 407 functions the stack tracer will check. 408 409 trace_clock: 410 411 Whenever an event is recorded into the ring buffer, a 412 "timestamp" is added. This stamp comes from a specified 413 clock. By default, ftrace uses the "local" clock. This 414 clock is very fast and strictly per cpu, but on some 415 systems it may not be monotonic with respect to other 416 CPUs. In other words, the local clocks may not be in sync 417 with local clocks on other CPUs. 418 419 Usual clocks for tracing:: 420 421 # cat trace_clock 422 [local] global counter x86-tsc 423 424 The clock with the square brackets around it is the one in effect. 425 426 local: 427 Default clock, but may not be in sync across CPUs 428 429 global: 430 This clock is in sync with all CPUs but may 431 be a bit slower than the local clock. 432 433 counter: 434 This is not a clock at all, but literally an atomic 435 counter. It counts up one by one, but is in sync 436 with all CPUs. This is useful when you need to 437 know exactly the order events occurred with respect to 438 each other on different CPUs. 439 440 uptime: 441 This uses the jiffies counter and the time stamp 442 is relative to the time since boot up. 443 444 perf: 445 This makes ftrace use the same clock that perf uses. 446 Eventually perf will be able to read ftrace buffers 447 and this will help out in interleaving the data. 448 449 x86-tsc: 450 Architectures may define their own clocks. For 451 example, x86 uses its own TSC cycle clock here. 452 453 ppc-tb: 454 This uses the powerpc timebase register value. 455 This is in sync across CPUs and can also be used 456 to correlate events across hypervisor/guest if 457 tb_offset is known. 458 459 mono: 460 This uses the fast monotonic clock (CLOCK_MONOTONIC) 461 which is monotonic and is subject to NTP rate adjustments. 462 463 mono_raw: 464 This is the raw monotonic clock (CLOCK_MONOTONIC_RAW) 465 which is monotonic but is not subject to any rate adjustments 466 and ticks at the same rate as the hardware clocksource. 467 468 boot: 469 This is the boot clock (CLOCK_BOOTTIME) and is based on the 470 fast monotonic clock, but also accounts for time spent in 471 suspend. Since the clock access is designed for use in 472 tracing in the suspend path, some side effects are possible 473 if clock is accessed after the suspend time is accounted before 474 the fast mono clock is updated. In this case, the clock update 475 appears to happen slightly sooner than it normally would have. 476 Also on 32-bit systems, it's possible that the 64-bit boot offset 477 sees a partial update. These effects are rare and post 478 processing should be able to handle them. See comments in the 479 ktime_get_boot_fast_ns() function for more information. 480 481 To set a clock, simply echo the clock name into this file:: 482 483 # echo global > trace_clock 484 485 trace_marker: 486 487 This is a very useful file for synchronizing user space 488 with events happening in the kernel. Writing strings into 489 this file will be written into the ftrace buffer. 490 491 It is useful in applications to open this file at the start 492 of the application and just reference the file descriptor 493 for the file:: 494 495 void trace_write(const char *fmt, ...) 496 { 497 va_list ap; 498 char buf[256]; 499 int n; 500 501 if (trace_fd < 0) 502 return; 503 504 va_start(ap, fmt); 505 n = vsnprintf(buf, 256, fmt, ap); 506 va_end(ap); 507 508 write(trace_fd, buf, n); 509 } 510 511 start:: 512 513 trace_fd = open("trace_marker", WR_ONLY); 514 515 Note: Writing into the trace_marker file can also initiate triggers 516 that are written into /sys/kernel/tracing/events/ftrace/print/trigger 517 See "Event triggers" in Documentation/trace/events.rst and an 518 example in Documentation/trace/histogram.rst (Section 3.) 519 520 trace_marker_raw: 521 522 This is similar to trace_marker above, but is meant for for binary data 523 to be written to it, where a tool can be used to parse the data 524 from trace_pipe_raw. 525 526 uprobe_events: 527 528 Add dynamic tracepoints in programs. 529 See uprobetracer.txt 530 531 uprobe_profile: 532 533 Uprobe statistics. See uprobetrace.txt 534 535 instances: 536 537 This is a way to make multiple trace buffers where different 538 events can be recorded in different buffers. 539 See "Instances" section below. 540 541 events: 542 543 This is the trace event directory. It holds event tracepoints 544 (also known as static tracepoints) that have been compiled 545 into the kernel. It shows what event tracepoints exist 546 and how they are grouped by system. There are "enable" 547 files at various levels that can enable the tracepoints 548 when a "1" is written to them. 549 550 See events.txt for more information. 551 552 set_event: 553 554 By echoing in the event into this file, will enable that event. 555 556 See events.txt for more information. 557 558 available_events: 559 560 A list of events that can be enabled in tracing. 561 562 See events.txt for more information. 563 564 timestamp_mode: 565 566 Certain tracers may change the timestamp mode used when 567 logging trace events into the event buffer. Events with 568 different modes can coexist within a buffer but the mode in 569 effect when an event is logged determines which timestamp mode 570 is used for that event. The default timestamp mode is 571 'delta'. 572 573 Usual timestamp modes for tracing: 574 575 # cat timestamp_mode 576 [delta] absolute 577 578 The timestamp mode with the square brackets around it is the 579 one in effect. 580 581 delta: Default timestamp mode - timestamp is a delta against 582 a per-buffer timestamp. 583 584 absolute: The timestamp is a full timestamp, not a delta 585 against some other value. As such it takes up more 586 space and is less efficient. 587 588 hwlat_detector: 589 590 Directory for the Hardware Latency Detector. 591 See "Hardware Latency Detector" section below. 592 593 per_cpu: 594 595 This is a directory that contains the trace per_cpu information. 596 597 per_cpu/cpu0/buffer_size_kb: 598 599 The ftrace buffer is defined per_cpu. That is, there's a separate 600 buffer for each CPU to allow writes to be done atomically, 601 and free from cache bouncing. These buffers may have different 602 size buffers. This file is similar to the buffer_size_kb 603 file, but it only displays or sets the buffer size for the 604 specific CPU. (here cpu0). 605 606 per_cpu/cpu0/trace: 607 608 This is similar to the "trace" file, but it will only display 609 the data specific for the CPU. If written to, it only clears 610 the specific CPU buffer. 611 612 per_cpu/cpu0/trace_pipe 613 614 This is similar to the "trace_pipe" file, and is a consuming 615 read, but it will only display (and consume) the data specific 616 for the CPU. 617 618 per_cpu/cpu0/trace_pipe_raw 619 620 For tools that can parse the ftrace ring buffer binary format, 621 the trace_pipe_raw file can be used to extract the data 622 from the ring buffer directly. With the use of the splice() 623 system call, the buffer data can be quickly transferred to 624 a file or to the network where a server is collecting the 625 data. 626 627 Like trace_pipe, this is a consuming reader, where multiple 628 reads will always produce different data. 629 630 per_cpu/cpu0/snapshot: 631 632 This is similar to the main "snapshot" file, but will only 633 snapshot the current CPU (if supported). It only displays 634 the content of the snapshot for a given CPU, and if 635 written to, only clears this CPU buffer. 636 637 per_cpu/cpu0/snapshot_raw: 638 639 Similar to the trace_pipe_raw, but will read the binary format 640 from the snapshot buffer for the given CPU. 641 642 per_cpu/cpu0/stats: 643 644 This displays certain stats about the ring buffer: 645 646 entries: 647 The number of events that are still in the buffer. 648 649 overrun: 650 The number of lost events due to overwriting when 651 the buffer was full. 652 653 commit overrun: 654 Should always be zero. 655 This gets set if so many events happened within a nested 656 event (ring buffer is re-entrant), that it fills the 657 buffer and starts dropping events. 658 659 bytes: 660 Bytes actually read (not overwritten). 661 662 oldest event ts: 663 The oldest timestamp in the buffer 664 665 now ts: 666 The current timestamp 667 668 dropped events: 669 Events lost due to overwrite option being off. 670 671 read events: 672 The number of events read. 673 674The Tracers 675----------- 676 677Here is the list of current tracers that may be configured. 678 679 "function" 680 681 Function call tracer to trace all kernel functions. 682 683 "function_graph" 684 685 Similar to the function tracer except that the 686 function tracer probes the functions on their entry 687 whereas the function graph tracer traces on both entry 688 and exit of the functions. It then provides the ability 689 to draw a graph of function calls similar to C code 690 source. 691 692 "blk" 693 694 The block tracer. The tracer used by the blktrace user 695 application. 696 697 "hwlat" 698 699 The Hardware Latency tracer is used to detect if the hardware 700 produces any latency. See "Hardware Latency Detector" section 701 below. 702 703 "irqsoff" 704 705 Traces the areas that disable interrupts and saves 706 the trace with the longest max latency. 707 See tracing_max_latency. When a new max is recorded, 708 it replaces the old trace. It is best to view this 709 trace with the latency-format option enabled, which 710 happens automatically when the tracer is selected. 711 712 "preemptoff" 713 714 Similar to irqsoff but traces and records the amount of 715 time for which preemption is disabled. 716 717 "preemptirqsoff" 718 719 Similar to irqsoff and preemptoff, but traces and 720 records the largest time for which irqs and/or preemption 721 is disabled. 722 723 "wakeup" 724 725 Traces and records the max latency that it takes for 726 the highest priority task to get scheduled after 727 it has been woken up. 728 Traces all tasks as an average developer would expect. 729 730 "wakeup_rt" 731 732 Traces and records the max latency that it takes for just 733 RT tasks (as the current "wakeup" does). This is useful 734 for those interested in wake up timings of RT tasks. 735 736 "wakeup_dl" 737 738 Traces and records the max latency that it takes for 739 a SCHED_DEADLINE task to be woken (as the "wakeup" and 740 "wakeup_rt" does). 741 742 "mmiotrace" 743 744 A special tracer that is used to trace binary module. 745 It will trace all the calls that a module makes to the 746 hardware. Everything it writes and reads from the I/O 747 as well. 748 749 "branch" 750 751 This tracer can be configured when tracing likely/unlikely 752 calls within the kernel. It will trace when a likely and 753 unlikely branch is hit and if it was correct in its prediction 754 of being correct. 755 756 "nop" 757 758 This is the "trace nothing" tracer. To remove all 759 tracers from tracing simply echo "nop" into 760 current_tracer. 761 762 763Examples of using the tracer 764---------------------------- 765 766Here are typical examples of using the tracers when controlling 767them only with the tracefs interface (without using any 768user-land utilities). 769 770Output format: 771-------------- 772 773Here is an example of the output format of the file "trace":: 774 775 # tracer: function 776 # 777 # entries-in-buffer/entries-written: 140080/250280 #P:4 778 # 779 # _-----=> irqs-off 780 # / _----=> need-resched 781 # | / _---=> hardirq/softirq 782 # || / _--=> preempt-depth 783 # ||| / delay 784 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 785 # | | | |||| | | 786 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath 787 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close 788 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd 789 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify 790 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock 791 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd 792 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock 793 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd 794 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close 795 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath 796 .... 797 798A header is printed with the tracer name that is represented by 799the trace. In this case the tracer is "function". Then it shows the 800number of events in the buffer as well as the total number of entries 801that were written. The difference is the number of entries that were 802lost due to the buffer filling up (250280 - 140080 = 110200 events 803lost). 804 805The header explains the content of the events. Task name "bash", the task 806PID "1977", the CPU that it was running on "000", the latency format 807(explained below), the timestamp in <secs>.<usecs> format, the 808function name that was traced "sys_close" and the parent function that 809called this function "system_call_fastpath". The timestamp is the time 810at which the function was entered. 811 812Latency trace format 813-------------------- 814 815When the latency-format option is enabled or when one of the latency 816tracers is set, the trace file gives somewhat more information to see 817why a latency happened. Here is a typical trace:: 818 819 # tracer: irqsoff 820 # 821 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 822 # -------------------------------------------------------------------- 823 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 824 # ----------------- 825 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0) 826 # ----------------- 827 # => started at: __lock_task_sighand 828 # => ended at: _raw_spin_unlock_irqrestore 829 # 830 # 831 # _------=> CPU# 832 # / _-----=> irqs-off 833 # | / _----=> need-resched 834 # || / _---=> hardirq/softirq 835 # ||| / _--=> preempt-depth 836 # |||| / delay 837 # cmd pid ||||| time | caller 838 # \ / ||||| \ | / 839 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand 840 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore 841 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore 842 ps-6143 2d..1 306us : <stack trace> 843 => trace_hardirqs_on_caller 844 => trace_hardirqs_on 845 => _raw_spin_unlock_irqrestore 846 => do_task_stat 847 => proc_tgid_stat 848 => proc_single_show 849 => seq_read 850 => vfs_read 851 => sys_read 852 => system_call_fastpath 853 854 855This shows that the current tracer is "irqsoff" tracing the time 856for which interrupts were disabled. It gives the trace version (which 857never changes) and the version of the kernel upon which this was executed on 858(3.8). Then it displays the max latency in microseconds (259 us). The number 859of trace entries displayed and the total number (both are four: #4/4). 860VP, KP, SP, and HP are always zero and are reserved for later use. 861#P is the number of online CPUs (#P:4). 862 863The task is the process that was running when the latency 864occurred. (ps pid: 6143). 865 866The start and stop (the functions in which the interrupts were 867disabled and enabled respectively) that caused the latencies: 868 869 - __lock_task_sighand is where the interrupts were disabled. 870 - _raw_spin_unlock_irqrestore is where they were enabled again. 871 872The next lines after the header are the trace itself. The header 873explains which is which. 874 875 cmd: The name of the process in the trace. 876 877 pid: The PID of that process. 878 879 CPU#: The CPU which the process was running on. 880 881 irqs-off: 'd' interrupts are disabled. '.' otherwise. 882 .. caution:: If the architecture does not support a way to 883 read the irq flags variable, an 'X' will always 884 be printed here. 885 886 need-resched: 887 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set, 888 - 'n' only TIF_NEED_RESCHED is set, 889 - 'p' only PREEMPT_NEED_RESCHED is set, 890 - '.' otherwise. 891 892 hardirq/softirq: 893 - 'Z' - NMI occurred inside a hardirq 894 - 'z' - NMI is running 895 - 'H' - hard irq occurred inside a softirq. 896 - 'h' - hard irq is running 897 - 's' - soft irq is running 898 - '.' - normal context. 899 900 preempt-depth: The level of preempt_disabled 901 902The above is mostly meaningful for kernel developers. 903 904 time: 905 When the latency-format option is enabled, the trace file 906 output includes a timestamp relative to the start of the 907 trace. This differs from the output when latency-format 908 is disabled, which includes an absolute timestamp. 909 910 delay: 911 This is just to help catch your eye a bit better. And 912 needs to be fixed to be only relative to the same CPU. 913 The marks are determined by the difference between this 914 current trace and the next trace. 915 916 - '$' - greater than 1 second 917 - '@' - greater than 100 millisecond 918 - '*' - greater than 10 millisecond 919 - '#' - greater than 1000 microsecond 920 - '!' - greater than 100 microsecond 921 - '+' - greater than 10 microsecond 922 - ' ' - less than or equal to 10 microsecond. 923 924 The rest is the same as the 'trace' file. 925 926 Note, the latency tracers will usually end with a back trace 927 to easily find where the latency occurred. 928 929trace_options 930------------- 931 932The trace_options file (or the options directory) is used to control 933what gets printed in the trace output, or manipulate the tracers. 934To see what is available, simply cat the file:: 935 936 cat trace_options 937 print-parent 938 nosym-offset 939 nosym-addr 940 noverbose 941 noraw 942 nohex 943 nobin 944 noblock 945 trace_printk 946 annotate 947 nouserstacktrace 948 nosym-userobj 949 noprintk-msg-only 950 context-info 951 nolatency-format 952 record-cmd 953 norecord-tgid 954 overwrite 955 nodisable_on_free 956 irq-info 957 markers 958 noevent-fork 959 function-trace 960 nofunction-fork 961 nodisplay-graph 962 nostacktrace 963 nobranch 964 965To disable one of the options, echo in the option prepended with 966"no":: 967 968 echo noprint-parent > trace_options 969 970To enable an option, leave off the "no":: 971 972 echo sym-offset > trace_options 973 974Here are the available options: 975 976 print-parent 977 On function traces, display the calling (parent) 978 function as well as the function being traced. 979 :: 980 981 print-parent: 982 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul 983 984 noprint-parent: 985 bash-4000 [01] 1477.606694: simple_strtoul 986 987 988 sym-offset 989 Display not only the function name, but also the 990 offset in the function. For example, instead of 991 seeing just "ktime_get", you will see 992 "ktime_get+0xb/0x20". 993 :: 994 995 sym-offset: 996 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 997 998 sym-addr 999 This will also display the function address as well 1000 as the function name. 1001 :: 1002 1003 sym-addr: 1004 bash-4000 [01] 1477.606694: simple_strtoul <c0339346> 1005 1006 verbose 1007 This deals with the trace file when the 1008 latency-format option is enabled. 1009 :: 1010 1011 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ 1012 (+0.000ms): simple_strtoul (kstrtoul) 1013 1014 raw 1015 This will display raw numbers. This option is best for 1016 use with user applications that can translate the raw 1017 numbers better than having it done in the kernel. 1018 1019 hex 1020 Similar to raw, but the numbers will be in a hexadecimal format. 1021 1022 bin 1023 This will print out the formats in raw binary. 1024 1025 block 1026 When set, reading trace_pipe will not block when polled. 1027 1028 trace_printk 1029 Can disable trace_printk() from writing into the buffer. 1030 1031 annotate 1032 It is sometimes confusing when the CPU buffers are full 1033 and one CPU buffer had a lot of events recently, thus 1034 a shorter time frame, were another CPU may have only had 1035 a few events, which lets it have older events. When 1036 the trace is reported, it shows the oldest events first, 1037 and it may look like only one CPU ran (the one with the 1038 oldest events). When the annotate option is set, it will 1039 display when a new CPU buffer started:: 1040 1041 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on 1042 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on 1043 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore 1044 ##### CPU 2 buffer started #### 1045 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle 1046 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog 1047 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock 1048 1049 userstacktrace 1050 This option changes the trace. It records a 1051 stacktrace of the current user space thread after 1052 each trace event. 1053 1054 sym-userobj 1055 when user stacktrace are enabled, look up which 1056 object the address belongs to, and print a 1057 relative address. This is especially useful when 1058 ASLR is on, otherwise you don't get a chance to 1059 resolve the address to object/file/line after 1060 the app is no longer running 1061 1062 The lookup is performed when you read 1063 trace,trace_pipe. Example:: 1064 1065 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 1066 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] 1067 1068 1069 printk-msg-only 1070 When set, trace_printk()s will only show the format 1071 and not their parameters (if trace_bprintk() or 1072 trace_bputs() was used to save the trace_printk()). 1073 1074 context-info 1075 Show only the event data. Hides the comm, PID, 1076 timestamp, CPU, and other useful data. 1077 1078 latency-format 1079 This option changes the trace output. When it is enabled, 1080 the trace displays additional information about the 1081 latency, as described in "Latency trace format". 1082 1083 record-cmd 1084 When any event or tracer is enabled, a hook is enabled 1085 in the sched_switch trace point to fill comm cache 1086 with mapped pids and comms. But this may cause some 1087 overhead, and if you only care about pids, and not the 1088 name of the task, disabling this option can lower the 1089 impact of tracing. See "saved_cmdlines". 1090 1091 record-tgid 1092 When any event or tracer is enabled, a hook is enabled 1093 in the sched_switch trace point to fill the cache of 1094 mapped Thread Group IDs (TGID) mapping to pids. See 1095 "saved_tgids". 1096 1097 overwrite 1098 This controls what happens when the trace buffer is 1099 full. If "1" (default), the oldest events are 1100 discarded and overwritten. If "0", then the newest 1101 events are discarded. 1102 (see per_cpu/cpu0/stats for overrun and dropped) 1103 1104 disable_on_free 1105 When the free_buffer is closed, tracing will 1106 stop (tracing_on set to 0). 1107 1108 irq-info 1109 Shows the interrupt, preempt count, need resched data. 1110 When disabled, the trace looks like:: 1111 1112 # tracer: function 1113 # 1114 # entries-in-buffer/entries-written: 144405/9452052 #P:4 1115 # 1116 # TASK-PID CPU# TIMESTAMP FUNCTION 1117 # | | | | | 1118 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up 1119 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89 1120 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task 1121 1122 1123 markers 1124 When set, the trace_marker is writable (only by root). 1125 When disabled, the trace_marker will error with EINVAL 1126 on write. 1127 1128 event-fork 1129 When set, tasks with PIDs listed in set_event_pid will have 1130 the PIDs of their children added to set_event_pid when those 1131 tasks fork. Also, when tasks with PIDs in set_event_pid exit, 1132 their PIDs will be removed from the file. 1133 1134 function-trace 1135 The latency tracers will enable function tracing 1136 if this option is enabled (default it is). When 1137 it is disabled, the latency tracers do not trace 1138 functions. This keeps the overhead of the tracer down 1139 when performing latency tests. 1140 1141 function-fork 1142 When set, tasks with PIDs listed in set_ftrace_pid will 1143 have the PIDs of their children added to set_ftrace_pid 1144 when those tasks fork. Also, when tasks with PIDs in 1145 set_ftrace_pid exit, their PIDs will be removed from the 1146 file. 1147 1148 display-graph 1149 When set, the latency tracers (irqsoff, wakeup, etc) will 1150 use function graph tracing instead of function tracing. 1151 1152 stacktrace 1153 When set, a stack trace is recorded after any trace event 1154 is recorded. 1155 1156 branch 1157 Enable branch tracing with the tracer. This enables branch 1158 tracer along with the currently set tracer. Enabling this 1159 with the "nop" tracer is the same as just enabling the 1160 "branch" tracer. 1161 1162.. tip:: Some tracers have their own options. They only appear in this 1163 file when the tracer is active. They always appear in the 1164 options directory. 1165 1166 1167Here are the per tracer options: 1168 1169Options for function tracer: 1170 1171 func_stack_trace 1172 When set, a stack trace is recorded after every 1173 function that is recorded. NOTE! Limit the functions 1174 that are recorded before enabling this, with 1175 "set_ftrace_filter" otherwise the system performance 1176 will be critically degraded. Remember to disable 1177 this option before clearing the function filter. 1178 1179Options for function_graph tracer: 1180 1181 Since the function_graph tracer has a slightly different output 1182 it has its own options to control what is displayed. 1183 1184 funcgraph-overrun 1185 When set, the "overrun" of the graph stack is 1186 displayed after each function traced. The 1187 overrun, is when the stack depth of the calls 1188 is greater than what is reserved for each task. 1189 Each task has a fixed array of functions to 1190 trace in the call graph. If the depth of the 1191 calls exceeds that, the function is not traced. 1192 The overrun is the number of functions missed 1193 due to exceeding this array. 1194 1195 funcgraph-cpu 1196 When set, the CPU number of the CPU where the trace 1197 occurred is displayed. 1198 1199 funcgraph-overhead 1200 When set, if the function takes longer than 1201 A certain amount, then a delay marker is 1202 displayed. See "delay" above, under the 1203 header description. 1204 1205 funcgraph-proc 1206 Unlike other tracers, the process' command line 1207 is not displayed by default, but instead only 1208 when a task is traced in and out during a context 1209 switch. Enabling this options has the command 1210 of each process displayed at every line. 1211 1212 funcgraph-duration 1213 At the end of each function (the return) 1214 the duration of the amount of time in the 1215 function is displayed in microseconds. 1216 1217 funcgraph-abstime 1218 When set, the timestamp is displayed at each line. 1219 1220 funcgraph-irqs 1221 When disabled, functions that happen inside an 1222 interrupt will not be traced. 1223 1224 funcgraph-tail 1225 When set, the return event will include the function 1226 that it represents. By default this is off, and 1227 only a closing curly bracket "}" is displayed for 1228 the return of a function. 1229 1230 sleep-time 1231 When running function graph tracer, to include 1232 the time a task schedules out in its function. 1233 When enabled, it will account time the task has been 1234 scheduled out as part of the function call. 1235 1236 graph-time 1237 When running function profiler with function graph tracer, 1238 to include the time to call nested functions. When this is 1239 not set, the time reported for the function will only 1240 include the time the function itself executed for, not the 1241 time for functions that it called. 1242 1243Options for blk tracer: 1244 1245 blk_classic 1246 Shows a more minimalistic output. 1247 1248 1249irqsoff 1250------- 1251 1252When interrupts are disabled, the CPU can not react to any other 1253external event (besides NMIs and SMIs). This prevents the timer 1254interrupt from triggering or the mouse interrupt from letting 1255the kernel know of a new mouse event. The result is a latency 1256with the reaction time. 1257 1258The irqsoff tracer tracks the time for which interrupts are 1259disabled. When a new maximum latency is hit, the tracer saves 1260the trace leading up to that latency point so that every time a 1261new maximum is reached, the old saved trace is discarded and the 1262new trace is saved. 1263 1264To reset the maximum, echo 0 into tracing_max_latency. Here is 1265an example:: 1266 1267 # echo 0 > options/function-trace 1268 # echo irqsoff > current_tracer 1269 # echo 1 > tracing_on 1270 # echo 0 > tracing_max_latency 1271 # ls -ltr 1272 [...] 1273 # echo 0 > tracing_on 1274 # cat trace 1275 # tracer: irqsoff 1276 # 1277 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1278 # -------------------------------------------------------------------- 1279 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1280 # ----------------- 1281 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0) 1282 # ----------------- 1283 # => started at: run_timer_softirq 1284 # => ended at: run_timer_softirq 1285 # 1286 # 1287 # _------=> CPU# 1288 # / _-----=> irqs-off 1289 # | / _----=> need-resched 1290 # || / _---=> hardirq/softirq 1291 # ||| / _--=> preempt-depth 1292 # |||| / delay 1293 # cmd pid ||||| time | caller 1294 # \ / ||||| \ | / 1295 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq 1296 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq 1297 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq 1298 <idle>-0 0dNs3 25us : <stack trace> 1299 => _raw_spin_unlock_irq 1300 => run_timer_softirq 1301 => __do_softirq 1302 => call_softirq 1303 => do_softirq 1304 => irq_exit 1305 => smp_apic_timer_interrupt 1306 => apic_timer_interrupt 1307 => rcu_idle_exit 1308 => cpu_idle 1309 => rest_init 1310 => start_kernel 1311 => x86_64_start_reservations 1312 => x86_64_start_kernel 1313 1314Here we see that that we had a latency of 16 microseconds (which is 1315very good). The _raw_spin_lock_irq in run_timer_softirq disabled 1316interrupts. The difference between the 16 and the displayed 1317timestamp 25us occurred because the clock was incremented 1318between the time of recording the max latency and the time of 1319recording the function that had that latency. 1320 1321Note the above example had function-trace not set. If we set 1322function-trace, we get a much larger output:: 1323 1324 with echo 1 > options/function-trace 1325 1326 # tracer: irqsoff 1327 # 1328 # irqsoff latency trace v1.1.5 on 3.8.0-test+ 1329 # -------------------------------------------------------------------- 1330 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1331 # ----------------- 1332 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0) 1333 # ----------------- 1334 # => started at: ata_scsi_queuecmd 1335 # => ended at: ata_scsi_queuecmd 1336 # 1337 # 1338 # _------=> CPU# 1339 # / _-----=> irqs-off 1340 # | / _----=> need-resched 1341 # || / _---=> hardirq/softirq 1342 # ||| / _--=> preempt-depth 1343 # |||| / delay 1344 # cmd pid ||||| time | caller 1345 # \ / ||||| \ | / 1346 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1347 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave 1348 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd 1349 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev 1350 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev 1351 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd 1352 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd 1353 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd 1354 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat 1355 [...] 1356 bash-2042 3d..1 67us : delay_tsc <-__delay 1357 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1358 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc 1359 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc 1360 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc 1361 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue 1362 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1363 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1364 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd 1365 bash-2042 3d..1 120us : <stack trace> 1366 => _raw_spin_unlock_irqrestore 1367 => ata_scsi_queuecmd 1368 => scsi_dispatch_cmd 1369 => scsi_request_fn 1370 => __blk_run_queue_uncond 1371 => __blk_run_queue 1372 => blk_queue_bio 1373 => generic_make_request 1374 => submit_bio 1375 => submit_bh 1376 => __ext3_get_inode_loc 1377 => ext3_iget 1378 => ext3_lookup 1379 => lookup_real 1380 => __lookup_hash 1381 => walk_component 1382 => lookup_last 1383 => path_lookupat 1384 => filename_lookup 1385 => user_path_at_empty 1386 => user_path_at 1387 => vfs_fstatat 1388 => vfs_stat 1389 => sys_newstat 1390 => system_call_fastpath 1391 1392 1393Here we traced a 71 microsecond latency. But we also see all the 1394functions that were called during that time. Note that by 1395enabling function tracing, we incur an added overhead. This 1396overhead may extend the latency times. But nevertheless, this 1397trace has provided some very helpful debugging information. 1398 1399 1400preemptoff 1401---------- 1402 1403When preemption is disabled, we may be able to receive 1404interrupts but the task cannot be preempted and a higher 1405priority task must wait for preemption to be enabled again 1406before it can preempt a lower priority task. 1407 1408The preemptoff tracer traces the places that disable preemption. 1409Like the irqsoff tracer, it records the maximum latency for 1410which preemption was disabled. The control of preemptoff tracer 1411is much like the irqsoff tracer. 1412:: 1413 1414 # echo 0 > options/function-trace 1415 # echo preemptoff > current_tracer 1416 # echo 1 > tracing_on 1417 # echo 0 > tracing_max_latency 1418 # ls -ltr 1419 [...] 1420 # echo 0 > tracing_on 1421 # cat trace 1422 # tracer: preemptoff 1423 # 1424 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1425 # -------------------------------------------------------------------- 1426 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1427 # ----------------- 1428 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0) 1429 # ----------------- 1430 # => started at: do_IRQ 1431 # => ended at: do_IRQ 1432 # 1433 # 1434 # _------=> CPU# 1435 # / _-----=> irqs-off 1436 # | / _----=> need-resched 1437 # || / _---=> hardirq/softirq 1438 # ||| / _--=> preempt-depth 1439 # |||| / delay 1440 # cmd pid ||||| time | caller 1441 # \ / ||||| \ | / 1442 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ 1443 sshd-1991 1d..1 46us : irq_exit <-do_IRQ 1444 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ 1445 sshd-1991 1d..1 52us : <stack trace> 1446 => sub_preempt_count 1447 => irq_exit 1448 => do_IRQ 1449 => ret_from_intr 1450 1451 1452This has some more changes. Preemption was disabled when an 1453interrupt came in (notice the 'h'), and was enabled on exit. 1454But we also see that interrupts have been disabled when entering 1455the preempt off section and leaving it (the 'd'). We do not know if 1456interrupts were enabled in the mean time or shortly after this 1457was over. 1458:: 1459 1460 # tracer: preemptoff 1461 # 1462 # preemptoff latency trace v1.1.5 on 3.8.0-test+ 1463 # -------------------------------------------------------------------- 1464 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1465 # ----------------- 1466 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0) 1467 # ----------------- 1468 # => started at: wake_up_new_task 1469 # => ended at: task_rq_unlock 1470 # 1471 # 1472 # _------=> CPU# 1473 # / _-----=> irqs-off 1474 # | / _----=> need-resched 1475 # || / _---=> hardirq/softirq 1476 # ||| / _--=> preempt-depth 1477 # |||| / delay 1478 # cmd pid ||||| time | caller 1479 # \ / ||||| \ | / 1480 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task 1481 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq 1482 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair 1483 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1484 bash-1994 1d..1 1us : source_load <-select_task_rq_fair 1485 [...] 1486 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt 1487 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter 1488 bash-1994 1d..1 13us : add_preempt_count <-irq_enter 1489 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt 1490 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt 1491 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt 1492 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock 1493 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt 1494 [...] 1495 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event 1496 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt 1497 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit 1498 bash-1994 1d..2 36us : do_softirq <-irq_exit 1499 bash-1994 1d..2 36us : __do_softirq <-call_softirq 1500 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq 1501 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq 1502 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq 1503 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock 1504 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq 1505 [...] 1506 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks 1507 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq 1508 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable 1509 bash-1994 1dN.2 82us : idle_cpu <-irq_exit 1510 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit 1511 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit 1512 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock 1513 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock 1514 bash-1994 1.N.1 104us : <stack trace> 1515 => sub_preempt_count 1516 => _raw_spin_unlock_irqrestore 1517 => task_rq_unlock 1518 => wake_up_new_task 1519 => do_fork 1520 => sys_clone 1521 => stub_clone 1522 1523 1524The above is an example of the preemptoff trace with 1525function-trace set. Here we see that interrupts were not disabled 1526the entire time. The irq_enter code lets us know that we entered 1527an interrupt 'h'. Before that, the functions being traced still 1528show that it is not in an interrupt, but we can see from the 1529functions themselves that this is not the case. 1530 1531preemptirqsoff 1532-------------- 1533 1534Knowing the locations that have interrupts disabled or 1535preemption disabled for the longest times is helpful. But 1536sometimes we would like to know when either preemption and/or 1537interrupts are disabled. 1538 1539Consider the following code:: 1540 1541 local_irq_disable(); 1542 call_function_with_irqs_off(); 1543 preempt_disable(); 1544 call_function_with_irqs_and_preemption_off(); 1545 local_irq_enable(); 1546 call_function_with_preemption_off(); 1547 preempt_enable(); 1548 1549The irqsoff tracer will record the total length of 1550call_function_with_irqs_off() and 1551call_function_with_irqs_and_preemption_off(). 1552 1553The preemptoff tracer will record the total length of 1554call_function_with_irqs_and_preemption_off() and 1555call_function_with_preemption_off(). 1556 1557But neither will trace the time that interrupts and/or 1558preemption is disabled. This total time is the time that we can 1559not schedule. To record this time, use the preemptirqsoff 1560tracer. 1561 1562Again, using this trace is much like the irqsoff and preemptoff 1563tracers. 1564:: 1565 1566 # echo 0 > options/function-trace 1567 # echo preemptirqsoff > current_tracer 1568 # echo 1 > tracing_on 1569 # echo 0 > tracing_max_latency 1570 # ls -ltr 1571 [...] 1572 # echo 0 > tracing_on 1573 # cat trace 1574 # tracer: preemptirqsoff 1575 # 1576 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1577 # -------------------------------------------------------------------- 1578 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1579 # ----------------- 1580 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0) 1581 # ----------------- 1582 # => started at: ata_scsi_queuecmd 1583 # => ended at: ata_scsi_queuecmd 1584 # 1585 # 1586 # _------=> CPU# 1587 # / _-----=> irqs-off 1588 # | / _----=> need-resched 1589 # || / _---=> hardirq/softirq 1590 # ||| / _--=> preempt-depth 1591 # |||| / delay 1592 # cmd pid ||||| time | caller 1593 # \ / ||||| \ | / 1594 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd 1595 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd 1596 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd 1597 ls-2230 3...1 111us : <stack trace> 1598 => sub_preempt_count 1599 => _raw_spin_unlock_irqrestore 1600 => ata_scsi_queuecmd 1601 => scsi_dispatch_cmd 1602 => scsi_request_fn 1603 => __blk_run_queue_uncond 1604 => __blk_run_queue 1605 => blk_queue_bio 1606 => generic_make_request 1607 => submit_bio 1608 => submit_bh 1609 => ext3_bread 1610 => ext3_dir_bread 1611 => htree_dirblock_to_tree 1612 => ext3_htree_fill_tree 1613 => ext3_readdir 1614 => vfs_readdir 1615 => sys_getdents 1616 => system_call_fastpath 1617 1618 1619The trace_hardirqs_off_thunk is called from assembly on x86 when 1620interrupts are disabled in the assembly code. Without the 1621function tracing, we do not know if interrupts were enabled 1622within the preemption points. We do see that it started with 1623preemption enabled. 1624 1625Here is a trace with function-trace set:: 1626 1627 # tracer: preemptirqsoff 1628 # 1629 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+ 1630 # -------------------------------------------------------------------- 1631 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1632 # ----------------- 1633 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0) 1634 # ----------------- 1635 # => started at: schedule 1636 # => ended at: mutex_unlock 1637 # 1638 # 1639 # _------=> CPU# 1640 # / _-----=> irqs-off 1641 # | / _----=> need-resched 1642 # || / _---=> hardirq/softirq 1643 # ||| / _--=> preempt-depth 1644 # |||| / delay 1645 # cmd pid ||||| time | caller 1646 # \ / ||||| \ | / 1647 kworker/-59 3...1 0us : __schedule <-schedule 1648 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch 1649 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq 1650 kworker/-59 3d..2 1us : deactivate_task <-__schedule 1651 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task 1652 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task 1653 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task 1654 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair 1655 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr 1656 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr 1657 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge 1658 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge 1659 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair 1660 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair 1661 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair 1662 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair 1663 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair 1664 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair 1665 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule 1666 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping 1667 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule 1668 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task 1669 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair 1670 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair 1671 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity 1672 ls-2269 3d..2 7us : finish_task_switch <-__schedule 1673 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch 1674 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr 1675 ls-2269 3d..2 8us : irq_enter <-do_IRQ 1676 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter 1677 ls-2269 3d..2 9us : add_preempt_count <-irq_enter 1678 ls-2269 3d.h2 9us : exit_idle <-do_IRQ 1679 [...] 1680 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock 1681 ls-2269 3d.h2 20us : irq_exit <-do_IRQ 1682 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit 1683 ls-2269 3d..3 21us : do_softirq <-irq_exit 1684 ls-2269 3d..3 21us : __do_softirq <-call_softirq 1685 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq 1686 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip 1687 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip 1688 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr 1689 ls-2269 3d.s5 31us : irq_enter <-do_IRQ 1690 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1691 [...] 1692 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter 1693 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter 1694 ls-2269 3d.H5 32us : exit_idle <-do_IRQ 1695 ls-2269 3d.H5 32us : handle_irq <-do_IRQ 1696 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq 1697 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq 1698 [...] 1699 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll 1700 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action 1701 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq 1702 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable 1703 ls-2269 3d..3 159us : idle_cpu <-irq_exit 1704 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit 1705 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit 1706 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock 1707 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock 1708 ls-2269 3d... 186us : <stack trace> 1709 => __mutex_unlock_slowpath 1710 => mutex_unlock 1711 => process_output 1712 => n_tty_write 1713 => tty_write 1714 => vfs_write 1715 => sys_write 1716 => system_call_fastpath 1717 1718This is an interesting trace. It started with kworker running and 1719scheduling out and ls taking over. But as soon as ls released the 1720rq lock and enabled interrupts (but not preemption) an interrupt 1721triggered. When the interrupt finished, it started running softirqs. 1722But while the softirq was running, another interrupt triggered. 1723When an interrupt is running inside a softirq, the annotation is 'H'. 1724 1725 1726wakeup 1727------ 1728 1729One common case that people are interested in tracing is the 1730time it takes for a task that is woken to actually wake up. 1731Now for non Real-Time tasks, this can be arbitrary. But tracing 1732it none the less can be interesting. 1733 1734Without function tracing:: 1735 1736 # echo 0 > options/function-trace 1737 # echo wakeup > current_tracer 1738 # echo 1 > tracing_on 1739 # echo 0 > tracing_max_latency 1740 # chrt -f 5 sleep 1 1741 # echo 0 > tracing_on 1742 # cat trace 1743 # tracer: wakeup 1744 # 1745 # wakeup latency trace v1.1.5 on 3.8.0-test+ 1746 # -------------------------------------------------------------------- 1747 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1748 # ----------------- 1749 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0) 1750 # ----------------- 1751 # 1752 # _------=> CPU# 1753 # / _-----=> irqs-off 1754 # | / _----=> need-resched 1755 # || / _---=> hardirq/softirq 1756 # ||| / _--=> preempt-depth 1757 # |||| / delay 1758 # cmd pid ||||| time | caller 1759 # \ / ||||| \ | / 1760 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H 1761 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 1762 <idle>-0 3d..3 15us : __schedule <-schedule 1763 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H 1764 1765The tracer only traces the highest priority task in the system 1766to avoid tracing the normal circumstances. Here we see that 1767the kworker with a nice priority of -20 (not very nice), took 1768just 15 microseconds from the time it woke up, to the time it 1769ran. 1770 1771Non Real-Time tasks are not that interesting. A more interesting 1772trace is to concentrate only on Real-Time tasks. 1773 1774wakeup_rt 1775--------- 1776 1777In a Real-Time environment it is very important to know the 1778wakeup time it takes for the highest priority task that is woken 1779up to the time that it executes. This is also known as "schedule 1780latency". I stress the point that this is about RT tasks. It is 1781also important to know the scheduling latency of non-RT tasks, 1782but the average schedule latency is better for non-RT tasks. 1783Tools like LatencyTop are more appropriate for such 1784measurements. 1785 1786Real-Time environments are interested in the worst case latency. 1787That is the longest latency it takes for something to happen, 1788and not the average. We can have a very fast scheduler that may 1789only have a large latency once in a while, but that would not 1790work well with Real-Time tasks. The wakeup_rt tracer was designed 1791to record the worst case wakeups of RT tasks. Non-RT tasks are 1792not recorded because the tracer only records one worst case and 1793tracing non-RT tasks that are unpredictable will overwrite the 1794worst case latency of RT tasks (just run the normal wakeup 1795tracer for a while to see that effect). 1796 1797Since this tracer only deals with RT tasks, we will run this 1798slightly differently than we did with the previous tracers. 1799Instead of performing an 'ls', we will run 'sleep 1' under 1800'chrt' which changes the priority of the task. 1801:: 1802 1803 # echo 0 > options/function-trace 1804 # echo wakeup_rt > current_tracer 1805 # echo 1 > tracing_on 1806 # echo 0 > tracing_max_latency 1807 # chrt -f 5 sleep 1 1808 # echo 0 > tracing_on 1809 # cat trace 1810 # tracer: wakeup 1811 # 1812 # tracer: wakeup_rt 1813 # 1814 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 1815 # -------------------------------------------------------------------- 1816 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1817 # ----------------- 1818 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5) 1819 # ----------------- 1820 # 1821 # _------=> CPU# 1822 # / _-----=> irqs-off 1823 # | / _----=> need-resched 1824 # || / _---=> hardirq/softirq 1825 # ||| / _--=> preempt-depth 1826 # |||| / delay 1827 # cmd pid ||||| time | caller 1828 # \ / ||||| \ | / 1829 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep 1830 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up 1831 <idle>-0 3d..3 5us : __schedule <-schedule 1832 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 1833 1834 1835Running this on an idle system, we see that it only took 5 microseconds 1836to perform the task switch. Note, since the trace point in the schedule 1837is before the actual "switch", we stop the tracing when the recorded task 1838is about to schedule in. This may change if we add a new marker at the 1839end of the scheduler. 1840 1841Notice that the recorded task is 'sleep' with the PID of 2389 1842and it has an rt_prio of 5. This priority is user-space priority 1843and not the internal kernel priority. The policy is 1 for 1844SCHED_FIFO and 2 for SCHED_RR. 1845 1846Note, that the trace data shows the internal priority (99 - rtprio). 1847:: 1848 1849 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep 1850 1851The 0:120:R means idle was running with a nice priority of 0 (120 - 120) 1852and in the running state 'R'. The sleep task was scheduled in with 18532389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94) 1854and it too is in the running state. 1855 1856Doing the same with chrt -r 5 and function-trace set. 1857:: 1858 1859 echo 1 > options/function-trace 1860 1861 # tracer: wakeup_rt 1862 # 1863 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 1864 # -------------------------------------------------------------------- 1865 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1866 # ----------------- 1867 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5) 1868 # ----------------- 1869 # 1870 # _------=> CPU# 1871 # / _-----=> irqs-off 1872 # | / _----=> need-resched 1873 # || / _---=> hardirq/softirq 1874 # ||| / _--=> preempt-depth 1875 # |||| / delay 1876 # cmd pid ||||| time | caller 1877 # \ / ||||| \ | / 1878 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep 1879 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up 1880 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup 1881 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr 1882 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup 1883 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up 1884 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock 1885 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up 1886 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up 1887 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore 1888 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer 1889 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock 1890 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt 1891 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock 1892 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt 1893 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event 1894 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event 1895 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event 1896 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt 1897 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit 1898 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit 1899 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit 1900 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit 1901 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit 1902 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle 1903 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit 1904 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle 1905 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit 1906 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit 1907 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit 1908 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit 1909 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz 1910 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock 1911 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz 1912 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update 1913 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz 1914 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock 1915 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit 1916 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit 1917 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit 1918 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel 1919 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel 1920 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 1921 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave 1922 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16 1923 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer 1924 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram 1925 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event 1926 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event 1927 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event 1928 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel 1929 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore 1930 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit 1931 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 1932 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward 1933 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 1934 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns 1935 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns 1936 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18 1937 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave 1938 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns 1939 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns 1940 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns 1941 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event 1942 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event 1943 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event 1944 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns 1945 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore 1946 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit 1947 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks 1948 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle 1949 <idle>-0 3.N.. 25us : schedule <-cpu_idle 1950 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule 1951 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule 1952 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule 1953 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch 1954 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch 1955 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule 1956 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq 1957 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule 1958 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task 1959 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task 1960 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt 1961 <idle>-0 3d..3 29us : __schedule <-preempt_schedule 1962 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep 1963 1964This isn't that big of a trace, even with function tracing enabled, 1965so I included the entire trace. 1966 1967The interrupt went off while when the system was idle. Somewhere 1968before task_woken_rt() was called, the NEED_RESCHED flag was set, 1969this is indicated by the first occurrence of the 'N' flag. 1970 1971Latency tracing and events 1972-------------------------- 1973As function tracing can induce a much larger latency, but without 1974seeing what happens within the latency it is hard to know what 1975caused it. There is a middle ground, and that is with enabling 1976events. 1977:: 1978 1979 # echo 0 > options/function-trace 1980 # echo wakeup_rt > current_tracer 1981 # echo 1 > events/enable 1982 # echo 1 > tracing_on 1983 # echo 0 > tracing_max_latency 1984 # chrt -f 5 sleep 1 1985 # echo 0 > tracing_on 1986 # cat trace 1987 # tracer: wakeup_rt 1988 # 1989 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+ 1990 # -------------------------------------------------------------------- 1991 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) 1992 # ----------------- 1993 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5) 1994 # ----------------- 1995 # 1996 # _------=> CPU# 1997 # / _-----=> irqs-off 1998 # | / _----=> need-resched 1999 # || / _---=> hardirq/softirq 2000 # ||| / _--=> preempt-depth 2001 # |||| / delay 2002 # cmd pid ||||| time | caller 2003 # \ / ||||| \ | / 2004 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep 2005 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up 2006 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002 2007 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8 2008 <idle>-0 2.N.2 2us : power_end: cpu_id=2 2009 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2 2010 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0 2011 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000 2012 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch 2013 <idle>-0 2.N.2 5us : rcu_utilization: End context switch 2014 <idle>-0 2d..3 6us : __schedule <-schedule 2015 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep 2016 2017 2018Hardware Latency Detector 2019------------------------- 2020 2021The hardware latency detector is executed by enabling the "hwlat" tracer. 2022 2023NOTE, this tracer will affect the performance of the system as it will 2024periodically make a CPU constantly busy with interrupts disabled. 2025:: 2026 2027 # echo hwlat > current_tracer 2028 # sleep 100 2029 # cat trace 2030 # tracer: hwlat 2031 # 2032 # _-----=> irqs-off 2033 # / _----=> need-resched 2034 # | / _---=> hardirq/softirq 2035 # || / _--=> preempt-depth 2036 # ||| / delay 2037 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2038 # | | | |||| | | 2039 <...>-3638 [001] d... 19452.055471: #1 inner/outer(us): 12/14 ts:1499801089.066141940 2040 <...>-3638 [003] d... 19454.071354: #2 inner/outer(us): 11/9 ts:1499801091.082164365 2041 <...>-3638 [002] dn.. 19461.126852: #3 inner/outer(us): 12/9 ts:1499801098.138150062 2042 <...>-3638 [001] d... 19488.340960: #4 inner/outer(us): 8/12 ts:1499801125.354139633 2043 <...>-3638 [003] d... 19494.388553: #5 inner/outer(us): 8/12 ts:1499801131.402150961 2044 <...>-3638 [003] d... 19501.283419: #6 inner/outer(us): 0/12 ts:1499801138.297435289 nmi-total:4 nmi-count:1 2045 2046 2047The above output is somewhat the same in the header. All events will have 2048interrupts disabled 'd'. Under the FUNCTION title there is: 2049 2050 #1 2051 This is the count of events recorded that were greater than the 2052 tracing_threshold (See below). 2053 2054 inner/outer(us): 12/14 2055 2056 This shows two numbers as "inner latency" and "outer latency". The test 2057 runs in a loop checking a timestamp twice. The latency detected within 2058 the two timestamps is the "inner latency" and the latency detected 2059 after the previous timestamp and the next timestamp in the loop is 2060 the "outer latency". 2061 2062 ts:1499801089.066141940 2063 2064 The absolute timestamp that the event happened. 2065 2066 nmi-total:4 nmi-count:1 2067 2068 On architectures that support it, if an NMI comes in during the 2069 test, the time spent in NMI is reported in "nmi-total" (in 2070 microseconds). 2071 2072 All architectures that have NMIs will show the "nmi-count" if an 2073 NMI comes in during the test. 2074 2075hwlat files: 2076 2077 tracing_threshold 2078 This gets automatically set to "10" to represent 10 2079 microseconds. This is the threshold of latency that 2080 needs to be detected before the trace will be recorded. 2081 2082 Note, when hwlat tracer is finished (another tracer is 2083 written into "current_tracer"), the original value for 2084 tracing_threshold is placed back into this file. 2085 2086 hwlat_detector/width 2087 The length of time the test runs with interrupts disabled. 2088 2089 hwlat_detector/window 2090 The length of time of the window which the test 2091 runs. That is, the test will run for "width" 2092 microseconds per "window" microseconds 2093 2094 tracing_cpumask 2095 When the test is started. A kernel thread is created that 2096 runs the test. This thread will alternate between CPUs 2097 listed in the tracing_cpumask between each period 2098 (one "window"). To limit the test to specific CPUs 2099 set the mask in this file to only the CPUs that the test 2100 should run on. 2101 2102function 2103-------- 2104 2105This tracer is the function tracer. Enabling the function tracer 2106can be done from the debug file system. Make sure the 2107ftrace_enabled is set; otherwise this tracer is a nop. 2108See the "ftrace_enabled" section below. 2109:: 2110 2111 # sysctl kernel.ftrace_enabled=1 2112 # echo function > current_tracer 2113 # echo 1 > tracing_on 2114 # usleep 1 2115 # echo 0 > tracing_on 2116 # cat trace 2117 # tracer: function 2118 # 2119 # entries-in-buffer/entries-written: 24799/24799 #P:4 2120 # 2121 # _-----=> irqs-off 2122 # / _----=> need-resched 2123 # | / _---=> hardirq/softirq 2124 # || / _--=> preempt-depth 2125 # ||| / delay 2126 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2127 # | | | |||| | | 2128 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write 2129 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock 2130 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify 2131 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify 2132 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify 2133 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock 2134 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock 2135 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify 2136 [...] 2137 2138 2139Note: function tracer uses ring buffers to store the above 2140entries. The newest data may overwrite the oldest data. 2141Sometimes using echo to stop the trace is not sufficient because 2142the tracing could have overwritten the data that you wanted to 2143record. For this reason, it is sometimes better to disable 2144tracing directly from a program. This allows you to stop the 2145tracing at the point that you hit the part that you are 2146interested in. To disable the tracing directly from a C program, 2147something like following code snippet can be used:: 2148 2149 int trace_fd; 2150 [...] 2151 int main(int argc, char *argv[]) { 2152 [...] 2153 trace_fd = open(tracing_file("tracing_on"), O_WRONLY); 2154 [...] 2155 if (condition_hit()) { 2156 write(trace_fd, "0", 1); 2157 } 2158 [...] 2159 } 2160 2161 2162Single thread tracing 2163--------------------- 2164 2165By writing into set_ftrace_pid you can trace a 2166single thread. For example:: 2167 2168 # cat set_ftrace_pid 2169 no pid 2170 # echo 3111 > set_ftrace_pid 2171 # cat set_ftrace_pid 2172 3111 2173 # echo function > current_tracer 2174 # cat trace | head 2175 # tracer: function 2176 # 2177 # TASK-PID CPU# TIMESTAMP FUNCTION 2178 # | | | | | 2179 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return 2180 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range 2181 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel 2182 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel 2183 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll 2184 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll 2185 # echo > set_ftrace_pid 2186 # cat trace |head 2187 # tracer: function 2188 # 2189 # TASK-PID CPU# TIMESTAMP FUNCTION 2190 # | | | | | 2191 ##### CPU 3 buffer started #### 2192 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait 2193 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry 2194 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry 2195 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit 2196 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit 2197 2198If you want to trace a function when executing, you could use 2199something like this simple program. 2200:: 2201 2202 #include <stdio.h> 2203 #include <stdlib.h> 2204 #include <sys/types.h> 2205 #include <sys/stat.h> 2206 #include <fcntl.h> 2207 #include <unistd.h> 2208 #include <string.h> 2209 2210 #define _STR(x) #x 2211 #define STR(x) _STR(x) 2212 #define MAX_PATH 256 2213 2214 const char *find_tracefs(void) 2215 { 2216 static char tracefs[MAX_PATH+1]; 2217 static int tracefs_found; 2218 char type[100]; 2219 FILE *fp; 2220 2221 if (tracefs_found) 2222 return tracefs; 2223 2224 if ((fp = fopen("/proc/mounts","r")) == NULL) { 2225 perror("/proc/mounts"); 2226 return NULL; 2227 } 2228 2229 while (fscanf(fp, "%*s %" 2230 STR(MAX_PATH) 2231 "s %99s %*s %*d %*d\n", 2232 tracefs, type) == 2) { 2233 if (strcmp(type, "tracefs") == 0) 2234 break; 2235 } 2236 fclose(fp); 2237 2238 if (strcmp(type, "tracefs") != 0) { 2239 fprintf(stderr, "tracefs not mounted"); 2240 return NULL; 2241 } 2242 2243 strcat(tracefs, "/tracing/"); 2244 tracefs_found = 1; 2245 2246 return tracefs; 2247 } 2248 2249 const char *tracing_file(const char *file_name) 2250 { 2251 static char trace_file[MAX_PATH+1]; 2252 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name); 2253 return trace_file; 2254 } 2255 2256 int main (int argc, char **argv) 2257 { 2258 if (argc < 1) 2259 exit(-1); 2260 2261 if (fork() > 0) { 2262 int fd, ffd; 2263 char line[64]; 2264 int s; 2265 2266 ffd = open(tracing_file("current_tracer"), O_WRONLY); 2267 if (ffd < 0) 2268 exit(-1); 2269 write(ffd, "nop", 3); 2270 2271 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); 2272 s = sprintf(line, "%d\n", getpid()); 2273 write(fd, line, s); 2274 2275 write(ffd, "function", 8); 2276 2277 close(fd); 2278 close(ffd); 2279 2280 execvp(argv[1], argv+1); 2281 } 2282 2283 return 0; 2284 } 2285 2286Or this simple script! 2287:: 2288 2289 #!/bin/bash 2290 2291 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts` 2292 echo nop > $tracefs/tracing/current_tracer 2293 echo 0 > $tracefs/tracing/tracing_on 2294 echo $$ > $tracefs/tracing/set_ftrace_pid 2295 echo function > $tracefs/tracing/current_tracer 2296 echo 1 > $tracefs/tracing/tracing_on 2297 exec "$@" 2298 2299 2300function graph tracer 2301--------------------------- 2302 2303This tracer is similar to the function tracer except that it 2304probes a function on its entry and its exit. This is done by 2305using a dynamically allocated stack of return addresses in each 2306task_struct. On function entry the tracer overwrites the return 2307address of each function traced to set a custom probe. Thus the 2308original return address is stored on the stack of return address 2309in the task_struct. 2310 2311Probing on both ends of a function leads to special features 2312such as: 2313 2314- measure of a function's time execution 2315- having a reliable call stack to draw function calls graph 2316 2317This tracer is useful in several situations: 2318 2319- you want to find the reason of a strange kernel behavior and 2320 need to see what happens in detail on any areas (or specific 2321 ones). 2322 2323- you are experiencing weird latencies but it's difficult to 2324 find its origin. 2325 2326- you want to find quickly which path is taken by a specific 2327 function 2328 2329- you just want to peek inside a working kernel and want to see 2330 what happens there. 2331 2332:: 2333 2334 # tracer: function_graph 2335 # 2336 # CPU DURATION FUNCTION CALLS 2337 # | | | | | | | 2338 2339 0) | sys_open() { 2340 0) | do_sys_open() { 2341 0) | getname() { 2342 0) | kmem_cache_alloc() { 2343 0) 1.382 us | __might_sleep(); 2344 0) 2.478 us | } 2345 0) | strncpy_from_user() { 2346 0) | might_fault() { 2347 0) 1.389 us | __might_sleep(); 2348 0) 2.553 us | } 2349 0) 3.807 us | } 2350 0) 7.876 us | } 2351 0) | alloc_fd() { 2352 0) 0.668 us | _spin_lock(); 2353 0) 0.570 us | expand_files(); 2354 0) 0.586 us | _spin_unlock(); 2355 2356 2357There are several columns that can be dynamically 2358enabled/disabled. You can use every combination of options you 2359want, depending on your needs. 2360 2361- The cpu number on which the function executed is default 2362 enabled. It is sometimes better to only trace one cpu (see 2363 tracing_cpu_mask file) or you might sometimes see unordered 2364 function calls while cpu tracing switch. 2365 2366 - hide: echo nofuncgraph-cpu > trace_options 2367 - show: echo funcgraph-cpu > trace_options 2368 2369- The duration (function's time of execution) is displayed on 2370 the closing bracket line of a function or on the same line 2371 than the current function in case of a leaf one. It is default 2372 enabled. 2373 2374 - hide: echo nofuncgraph-duration > trace_options 2375 - show: echo funcgraph-duration > trace_options 2376 2377- The overhead field precedes the duration field in case of 2378 reached duration thresholds. 2379 2380 - hide: echo nofuncgraph-overhead > trace_options 2381 - show: echo funcgraph-overhead > trace_options 2382 - depends on: funcgraph-duration 2383 2384 ie:: 2385 2386 3) # 1837.709 us | } /* __switch_to */ 2387 3) | finish_task_switch() { 2388 3) 0.313 us | _raw_spin_unlock_irq(); 2389 3) 3.177 us | } 2390 3) # 1889.063 us | } /* __schedule */ 2391 3) ! 140.417 us | } /* __schedule */ 2392 3) # 2034.948 us | } /* schedule */ 2393 3) * 33998.59 us | } /* schedule_preempt_disabled */ 2394 2395 [...] 2396 2397 1) 0.260 us | msecs_to_jiffies(); 2398 1) 0.313 us | __rcu_read_unlock(); 2399 1) + 61.770 us | } 2400 1) + 64.479 us | } 2401 1) 0.313 us | rcu_bh_qs(); 2402 1) 0.313 us | __local_bh_enable(); 2403 1) ! 217.240 us | } 2404 1) 0.365 us | idle_cpu(); 2405 1) | rcu_irq_exit() { 2406 1) 0.417 us | rcu_eqs_enter_common.isra.47(); 2407 1) 3.125 us | } 2408 1) ! 227.812 us | } 2409 1) ! 457.395 us | } 2410 1) @ 119760.2 us | } 2411 2412 [...] 2413 2414 2) | handle_IPI() { 2415 1) 6.979 us | } 2416 2) 0.417 us | scheduler_ipi(); 2417 1) 9.791 us | } 2418 1) + 12.917 us | } 2419 2) 3.490 us | } 2420 1) + 15.729 us | } 2421 1) + 18.542 us | } 2422 2) $ 3594274 us | } 2423 2424Flags:: 2425 2426 + means that the function exceeded 10 usecs. 2427 ! means that the function exceeded 100 usecs. 2428 # means that the function exceeded 1000 usecs. 2429 * means that the function exceeded 10 msecs. 2430 @ means that the function exceeded 100 msecs. 2431 $ means that the function exceeded 1 sec. 2432 2433 2434- The task/pid field displays the thread cmdline and pid which 2435 executed the function. It is default disabled. 2436 2437 - hide: echo nofuncgraph-proc > trace_options 2438 - show: echo funcgraph-proc > trace_options 2439 2440 ie:: 2441 2442 # tracer: function_graph 2443 # 2444 # CPU TASK/PID DURATION FUNCTION CALLS 2445 # | | | | | | | | | 2446 0) sh-4802 | | d_free() { 2447 0) sh-4802 | | call_rcu() { 2448 0) sh-4802 | | __call_rcu() { 2449 0) sh-4802 | 0.616 us | rcu_process_gp_end(); 2450 0) sh-4802 | 0.586 us | check_for_new_grace_period(); 2451 0) sh-4802 | 2.899 us | } 2452 0) sh-4802 | 4.040 us | } 2453 0) sh-4802 | 5.151 us | } 2454 0) sh-4802 | + 49.370 us | } 2455 2456 2457- The absolute time field is an absolute timestamp given by the 2458 system clock since it started. A snapshot of this time is 2459 given on each entry/exit of functions 2460 2461 - hide: echo nofuncgraph-abstime > trace_options 2462 - show: echo funcgraph-abstime > trace_options 2463 2464 ie:: 2465 2466 # 2467 # TIME CPU DURATION FUNCTION CALLS 2468 # | | | | | | | | 2469 360.774522 | 1) 0.541 us | } 2470 360.774522 | 1) 4.663 us | } 2471 360.774523 | 1) 0.541 us | __wake_up_bit(); 2472 360.774524 | 1) 6.796 us | } 2473 360.774524 | 1) 7.952 us | } 2474 360.774525 | 1) 9.063 us | } 2475 360.774525 | 1) 0.615 us | journal_mark_dirty(); 2476 360.774527 | 1) 0.578 us | __brelse(); 2477 360.774528 | 1) | reiserfs_prepare_for_journal() { 2478 360.774528 | 1) | unlock_buffer() { 2479 360.774529 | 1) | wake_up_bit() { 2480 360.774529 | 1) | bit_waitqueue() { 2481 360.774530 | 1) 0.594 us | __phys_addr(); 2482 2483 2484The function name is always displayed after the closing bracket 2485for a function if the start of that function is not in the 2486trace buffer. 2487 2488Display of the function name after the closing bracket may be 2489enabled for functions whose start is in the trace buffer, 2490allowing easier searching with grep for function durations. 2491It is default disabled. 2492 2493 - hide: echo nofuncgraph-tail > trace_options 2494 - show: echo funcgraph-tail > trace_options 2495 2496 Example with nofuncgraph-tail (default):: 2497 2498 0) | putname() { 2499 0) | kmem_cache_free() { 2500 0) 0.518 us | __phys_addr(); 2501 0) 1.757 us | } 2502 0) 2.861 us | } 2503 2504 Example with funcgraph-tail:: 2505 2506 0) | putname() { 2507 0) | kmem_cache_free() { 2508 0) 0.518 us | __phys_addr(); 2509 0) 1.757 us | } /* kmem_cache_free() */ 2510 0) 2.861 us | } /* putname() */ 2511 2512You can put some comments on specific functions by using 2513trace_printk() For example, if you want to put a comment inside 2514the __might_sleep() function, you just have to include 2515<linux/ftrace.h> and call trace_printk() inside __might_sleep():: 2516 2517 trace_printk("I'm a comment!\n") 2518 2519will produce:: 2520 2521 1) | __might_sleep() { 2522 1) | /* I'm a comment! */ 2523 1) 1.449 us | } 2524 2525 2526You might find other useful features for this tracer in the 2527following "dynamic ftrace" section such as tracing only specific 2528functions or tasks. 2529 2530dynamic ftrace 2531-------------- 2532 2533If CONFIG_DYNAMIC_FTRACE is set, the system will run with 2534virtually no overhead when function tracing is disabled. The way 2535this works is the mcount function call (placed at the start of 2536every kernel function, produced by the -pg switch in gcc), 2537starts of pointing to a simple return. (Enabling FTRACE will 2538include the -pg switch in the compiling of the kernel.) 2539 2540At compile time every C file object is run through the 2541recordmcount program (located in the scripts directory). This 2542program will parse the ELF headers in the C object to find all 2543the locations in the .text section that call mcount. Starting 2544with gcc version 4.6, the -mfentry has been added for x86, which 2545calls "__fentry__" instead of "mcount". Which is called before 2546the creation of the stack frame. 2547 2548Note, not all sections are traced. They may be prevented by either 2549a notrace, or blocked another way and all inline functions are not 2550traced. Check the "available_filter_functions" file to see what functions 2551can be traced. 2552 2553A section called "__mcount_loc" is created that holds 2554references to all the mcount/fentry call sites in the .text section. 2555The recordmcount program re-links this section back into the 2556original object. The final linking stage of the kernel will add all these 2557references into a single table. 2558 2559On boot up, before SMP is initialized, the dynamic ftrace code 2560scans this table and updates all the locations into nops. It 2561also records the locations, which are added to the 2562available_filter_functions list. Modules are processed as they 2563are loaded and before they are executed. When a module is 2564unloaded, it also removes its functions from the ftrace function 2565list. This is automatic in the module unload code, and the 2566module author does not need to worry about it. 2567 2568When tracing is enabled, the process of modifying the function 2569tracepoints is dependent on architecture. The old method is to use 2570kstop_machine to prevent races with the CPUs executing code being 2571modified (which can cause the CPU to do undesirable things, especially 2572if the modified code crosses cache (or page) boundaries), and the nops are 2573patched back to calls. But this time, they do not call mcount 2574(which is just a function stub). They now call into the ftrace 2575infrastructure. 2576 2577The new method of modifying the function tracepoints is to place 2578a breakpoint at the location to be modified, sync all CPUs, modify 2579the rest of the instruction not covered by the breakpoint. Sync 2580all CPUs again, and then remove the breakpoint with the finished 2581version to the ftrace call site. 2582 2583Some archs do not even need to monkey around with the synchronization, 2584and can just slap the new code on top of the old without any 2585problems with other CPUs executing it at the same time. 2586 2587One special side-effect to the recording of the functions being 2588traced is that we can now selectively choose which functions we 2589wish to trace and which ones we want the mcount calls to remain 2590as nops. 2591 2592Two files are used, one for enabling and one for disabling the 2593tracing of specified functions. They are: 2594 2595 set_ftrace_filter 2596 2597and 2598 2599 set_ftrace_notrace 2600 2601A list of available functions that you can add to these files is 2602listed in: 2603 2604 available_filter_functions 2605 2606:: 2607 2608 # cat available_filter_functions 2609 put_prev_task_idle 2610 kmem_cache_create 2611 pick_next_task_rt 2612 get_online_cpus 2613 pick_next_task_fair 2614 mutex_lock 2615 [...] 2616 2617If I am only interested in sys_nanosleep and hrtimer_interrupt:: 2618 2619 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter 2620 # echo function > current_tracer 2621 # echo 1 > tracing_on 2622 # usleep 1 2623 # echo 0 > tracing_on 2624 # cat trace 2625 # tracer: function 2626 # 2627 # entries-in-buffer/entries-written: 5/5 #P:4 2628 # 2629 # _-----=> irqs-off 2630 # / _----=> need-resched 2631 # | / _---=> hardirq/softirq 2632 # || / _--=> preempt-depth 2633 # ||| / delay 2634 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2635 # | | | |||| | | 2636 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath 2637 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt 2638 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 2639 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt 2640 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt 2641 2642To see which functions are being traced, you can cat the file: 2643:: 2644 2645 # cat set_ftrace_filter 2646 hrtimer_interrupt 2647 sys_nanosleep 2648 2649 2650Perhaps this is not enough. The filters also allow glob(7) matching. 2651 2652 ``<match>*`` 2653 will match functions that begin with <match> 2654 ``*<match>`` 2655 will match functions that end with <match> 2656 ``*<match>*`` 2657 will match functions that have <match> in it 2658 ``<match1>*<match2>`` 2659 will match functions that begin with <match1> and end with <match2> 2660 2661.. note:: 2662 It is better to use quotes to enclose the wild cards, 2663 otherwise the shell may expand the parameters into names 2664 of files in the local directory. 2665 2666:: 2667 2668 # echo 'hrtimer_*' > set_ftrace_filter 2669 2670Produces:: 2671 2672 # tracer: function 2673 # 2674 # entries-in-buffer/entries-written: 897/897 #P:4 2675 # 2676 # _-----=> irqs-off 2677 # / _----=> need-resched 2678 # | / _---=> hardirq/softirq 2679 # || / _--=> preempt-depth 2680 # ||| / delay 2681 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2682 # | | | |||| | | 2683 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit 2684 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel 2685 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer 2686 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit 2687 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11 2688 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt 2689 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter 2690 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem 2691 2692Notice that we lost the sys_nanosleep. 2693:: 2694 2695 # cat set_ftrace_filter 2696 hrtimer_run_queues 2697 hrtimer_run_pending 2698 hrtimer_init 2699 hrtimer_cancel 2700 hrtimer_try_to_cancel 2701 hrtimer_forward 2702 hrtimer_start 2703 hrtimer_reprogram 2704 hrtimer_force_reprogram 2705 hrtimer_get_next_event 2706 hrtimer_interrupt 2707 hrtimer_nanosleep 2708 hrtimer_wakeup 2709 hrtimer_get_remaining 2710 hrtimer_get_res 2711 hrtimer_init_sleeper 2712 2713 2714This is because the '>' and '>>' act just like they do in bash. 2715To rewrite the filters, use '>' 2716To append to the filters, use '>>' 2717 2718To clear out a filter so that all functions will be recorded 2719again:: 2720 2721 # echo > set_ftrace_filter 2722 # cat set_ftrace_filter 2723 # 2724 2725Again, now we want to append. 2726 2727:: 2728 2729 # echo sys_nanosleep > set_ftrace_filter 2730 # cat set_ftrace_filter 2731 sys_nanosleep 2732 # echo 'hrtimer_*' >> set_ftrace_filter 2733 # cat set_ftrace_filter 2734 hrtimer_run_queues 2735 hrtimer_run_pending 2736 hrtimer_init 2737 hrtimer_cancel 2738 hrtimer_try_to_cancel 2739 hrtimer_forward 2740 hrtimer_start 2741 hrtimer_reprogram 2742 hrtimer_force_reprogram 2743 hrtimer_get_next_event 2744 hrtimer_interrupt 2745 sys_nanosleep 2746 hrtimer_nanosleep 2747 hrtimer_wakeup 2748 hrtimer_get_remaining 2749 hrtimer_get_res 2750 hrtimer_init_sleeper 2751 2752 2753The set_ftrace_notrace prevents those functions from being 2754traced. 2755:: 2756 2757 # echo '*preempt*' '*lock*' > set_ftrace_notrace 2758 2759Produces:: 2760 2761 # tracer: function 2762 # 2763 # entries-in-buffer/entries-written: 39608/39608 #P:4 2764 # 2765 # _-----=> irqs-off 2766 # / _----=> need-resched 2767 # | / _---=> hardirq/softirq 2768 # || / _--=> preempt-depth 2769 # ||| / delay 2770 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 2771 # | | | |||| | | 2772 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open 2773 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last 2774 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last 2775 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check 2776 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement 2777 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action 2778 bash-1994 [000] .... 4342.324899: do_truncate <-do_last 2779 bash-1994 [000] .... 4342.324899: should_remove_suid <-do_truncate 2780 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate 2781 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change 2782 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time 2783 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time 2784 2785We can see that there's no more lock or preempt tracing. 2786 2787 2788Dynamic ftrace with the function graph tracer 2789--------------------------------------------- 2790 2791Although what has been explained above concerns both the 2792function tracer and the function-graph-tracer, there are some 2793special features only available in the function-graph tracer. 2794 2795If you want to trace only one function and all of its children, 2796you just have to echo its name into set_graph_function:: 2797 2798 echo __do_fault > set_graph_function 2799 2800will produce the following "expanded" trace of the __do_fault() 2801function:: 2802 2803 0) | __do_fault() { 2804 0) | filemap_fault() { 2805 0) | find_lock_page() { 2806 0) 0.804 us | find_get_page(); 2807 0) | __might_sleep() { 2808 0) 1.329 us | } 2809 0) 3.904 us | } 2810 0) 4.979 us | } 2811 0) 0.653 us | _spin_lock(); 2812 0) 0.578 us | page_add_file_rmap(); 2813 0) 0.525 us | native_set_pte_at(); 2814 0) 0.585 us | _spin_unlock(); 2815 0) | unlock_page() { 2816 0) 0.541 us | page_waitqueue(); 2817 0) 0.639 us | __wake_up_bit(); 2818 0) 2.786 us | } 2819 0) + 14.237 us | } 2820 0) | __do_fault() { 2821 0) | filemap_fault() { 2822 0) | find_lock_page() { 2823 0) 0.698 us | find_get_page(); 2824 0) | __might_sleep() { 2825 0) 1.412 us | } 2826 0) 3.950 us | } 2827 0) 5.098 us | } 2828 0) 0.631 us | _spin_lock(); 2829 0) 0.571 us | page_add_file_rmap(); 2830 0) 0.526 us | native_set_pte_at(); 2831 0) 0.586 us | _spin_unlock(); 2832 0) | unlock_page() { 2833 0) 0.533 us | page_waitqueue(); 2834 0) 0.638 us | __wake_up_bit(); 2835 0) 2.793 us | } 2836 0) + 14.012 us | } 2837 2838You can also expand several functions at once:: 2839 2840 echo sys_open > set_graph_function 2841 echo sys_close >> set_graph_function 2842 2843Now if you want to go back to trace all functions you can clear 2844this special filter via:: 2845 2846 echo > set_graph_function 2847 2848 2849ftrace_enabled 2850-------------- 2851 2852Note, the proc sysctl ftrace_enable is a big on/off switch for the 2853function tracer. By default it is enabled (when function tracing is 2854enabled in the kernel). If it is disabled, all function tracing is 2855disabled. This includes not only the function tracers for ftrace, but 2856also for any other uses (perf, kprobes, stack tracing, profiling, etc). 2857 2858Please disable this with care. 2859 2860This can be disable (and enabled) with:: 2861 2862 sysctl kernel.ftrace_enabled=0 2863 sysctl kernel.ftrace_enabled=1 2864 2865 or 2866 2867 echo 0 > /proc/sys/kernel/ftrace_enabled 2868 echo 1 > /proc/sys/kernel/ftrace_enabled 2869 2870 2871Filter commands 2872--------------- 2873 2874A few commands are supported by the set_ftrace_filter interface. 2875Trace commands have the following format:: 2876 2877 <function>:<command>:<parameter> 2878 2879The following commands are supported: 2880 2881- mod: 2882 This command enables function filtering per module. The 2883 parameter defines the module. For example, if only the write* 2884 functions in the ext3 module are desired, run: 2885 2886 echo 'write*:mod:ext3' > set_ftrace_filter 2887 2888 This command interacts with the filter in the same way as 2889 filtering based on function names. Thus, adding more functions 2890 in a different module is accomplished by appending (>>) to the 2891 filter file. Remove specific module functions by prepending 2892 '!':: 2893 2894 echo '!writeback*:mod:ext3' >> set_ftrace_filter 2895 2896 Mod command supports module globbing. Disable tracing for all 2897 functions except a specific module:: 2898 2899 echo '!*:mod:!ext3' >> set_ftrace_filter 2900 2901 Disable tracing for all modules, but still trace kernel:: 2902 2903 echo '!*:mod:*' >> set_ftrace_filter 2904 2905 Enable filter only for kernel:: 2906 2907 echo '*write*:mod:!*' >> set_ftrace_filter 2908 2909 Enable filter for module globbing:: 2910 2911 echo '*write*:mod:*snd*' >> set_ftrace_filter 2912 2913- traceon/traceoff: 2914 These commands turn tracing on and off when the specified 2915 functions are hit. The parameter determines how many times the 2916 tracing system is turned on and off. If unspecified, there is 2917 no limit. For example, to disable tracing when a schedule bug 2918 is hit the first 5 times, run:: 2919 2920 echo '__schedule_bug:traceoff:5' > set_ftrace_filter 2921 2922 To always disable tracing when __schedule_bug is hit:: 2923 2924 echo '__schedule_bug:traceoff' > set_ftrace_filter 2925 2926 These commands are cumulative whether or not they are appended 2927 to set_ftrace_filter. To remove a command, prepend it by '!' 2928 and drop the parameter:: 2929 2930 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter 2931 2932 The above removes the traceoff command for __schedule_bug 2933 that have a counter. To remove commands without counters:: 2934 2935 echo '!__schedule_bug:traceoff' > set_ftrace_filter 2936 2937- snapshot: 2938 Will cause a snapshot to be triggered when the function is hit. 2939 :: 2940 2941 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter 2942 2943 To only snapshot once: 2944 :: 2945 2946 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter 2947 2948 To remove the above commands:: 2949 2950 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter 2951 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter 2952 2953- enable_event/disable_event: 2954 These commands can enable or disable a trace event. Note, because 2955 function tracing callbacks are very sensitive, when these commands 2956 are registered, the trace point is activated, but disabled in 2957 a "soft" mode. That is, the tracepoint will be called, but 2958 just will not be traced. The event tracepoint stays in this mode 2959 as long as there's a command that triggers it. 2960 :: 2961 2962 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \ 2963 set_ftrace_filter 2964 2965 The format is:: 2966 2967 <function>:enable_event:<system>:<event>[:count] 2968 <function>:disable_event:<system>:<event>[:count] 2969 2970 To remove the events commands:: 2971 2972 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \ 2973 set_ftrace_filter 2974 echo '!schedule:disable_event:sched:sched_switch' > \ 2975 set_ftrace_filter 2976 2977- dump: 2978 When the function is hit, it will dump the contents of the ftrace 2979 ring buffer to the console. This is useful if you need to debug 2980 something, and want to dump the trace when a certain function 2981 is hit. Perhaps it's a function that is called before a triple 2982 fault happens and does not allow you to get a regular dump. 2983 2984- cpudump: 2985 When the function is hit, it will dump the contents of the ftrace 2986 ring buffer for the current CPU to the console. Unlike the "dump" 2987 command, it only prints out the contents of the ring buffer for the 2988 CPU that executed the function that triggered the dump. 2989 2990- stacktrace: 2991 When the function is hit, a stack trace is recorded. 2992 2993trace_pipe 2994---------- 2995 2996The trace_pipe outputs the same content as the trace file, but 2997the effect on the tracing is different. Every read from 2998trace_pipe is consumed. This means that subsequent reads will be 2999different. The trace is live. 3000:: 3001 3002 # echo function > current_tracer 3003 # cat trace_pipe > /tmp/trace.out & 3004 [1] 4153 3005 # echo 1 > tracing_on 3006 # usleep 1 3007 # echo 0 > tracing_on 3008 # cat trace 3009 # tracer: function 3010 # 3011 # entries-in-buffer/entries-written: 0/0 #P:4 3012 # 3013 # _-----=> irqs-off 3014 # / _----=> need-resched 3015 # | / _---=> hardirq/softirq 3016 # || / _--=> preempt-depth 3017 # ||| / delay 3018 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3019 # | | | |||| | | 3020 3021 # 3022 # cat /tmp/trace.out 3023 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write 3024 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock 3025 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify 3026 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify 3027 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify 3028 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock 3029 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock 3030 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify 3031 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath 3032 3033 3034Note, reading the trace_pipe file will block until more input is 3035added. 3036 3037trace entries 3038------------- 3039 3040Having too much or not enough data can be troublesome in 3041diagnosing an issue in the kernel. The file buffer_size_kb is 3042used to modify the size of the internal trace buffers. The 3043number listed is the number of entries that can be recorded per 3044CPU. To know the full size, multiply the number of possible CPUs 3045with the number of entries. 3046:: 3047 3048 # cat buffer_size_kb 3049 1408 (units kilobytes) 3050 3051Or simply read buffer_total_size_kb 3052:: 3053 3054 # cat buffer_total_size_kb 3055 5632 3056 3057To modify the buffer, simple echo in a number (in 1024 byte segments). 3058:: 3059 3060 # echo 10000 > buffer_size_kb 3061 # cat buffer_size_kb 3062 10000 (units kilobytes) 3063 3064It will try to allocate as much as possible. If you allocate too 3065much, it can cause Out-Of-Memory to trigger. 3066:: 3067 3068 # echo 1000000000000 > buffer_size_kb 3069 -bash: echo: write error: Cannot allocate memory 3070 # cat buffer_size_kb 3071 85 3072 3073The per_cpu buffers can be changed individually as well: 3074:: 3075 3076 # echo 10000 > per_cpu/cpu0/buffer_size_kb 3077 # echo 100 > per_cpu/cpu1/buffer_size_kb 3078 3079When the per_cpu buffers are not the same, the buffer_size_kb 3080at the top level will just show an X 3081:: 3082 3083 # cat buffer_size_kb 3084 X 3085 3086This is where the buffer_total_size_kb is useful: 3087:: 3088 3089 # cat buffer_total_size_kb 3090 12916 3091 3092Writing to the top level buffer_size_kb will reset all the buffers 3093to be the same again. 3094 3095Snapshot 3096-------- 3097CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature 3098available to all non latency tracers. (Latency tracers which 3099record max latency, such as "irqsoff" or "wakeup", can't use 3100this feature, since those are already using the snapshot 3101mechanism internally.) 3102 3103Snapshot preserves a current trace buffer at a particular point 3104in time without stopping tracing. Ftrace swaps the current 3105buffer with a spare buffer, and tracing continues in the new 3106current (=previous spare) buffer. 3107 3108The following tracefs files in "tracing" are related to this 3109feature: 3110 3111 snapshot: 3112 3113 This is used to take a snapshot and to read the output 3114 of the snapshot. Echo 1 into this file to allocate a 3115 spare buffer and to take a snapshot (swap), then read 3116 the snapshot from this file in the same format as 3117 "trace" (described above in the section "The File 3118 System"). Both reads snapshot and tracing are executable 3119 in parallel. When the spare buffer is allocated, echoing 3120 0 frees it, and echoing else (positive) values clear the 3121 snapshot contents. 3122 More details are shown in the table below. 3123 3124 +--------------+------------+------------+------------+ 3125 |status\\input | 0 | 1 | else | 3126 +==============+============+============+============+ 3127 |not allocated |(do nothing)| alloc+swap |(do nothing)| 3128 +--------------+------------+------------+------------+ 3129 |allocated | free | swap | clear | 3130 +--------------+------------+------------+------------+ 3131 3132Here is an example of using the snapshot feature. 3133:: 3134 3135 # echo 1 > events/sched/enable 3136 # echo 1 > snapshot 3137 # cat snapshot 3138 # tracer: nop 3139 # 3140 # entries-in-buffer/entries-written: 71/71 #P:8 3141 # 3142 # _-----=> irqs-off 3143 # / _----=> need-resched 3144 # | / _---=> hardirq/softirq 3145 # || / _--=> preempt-depth 3146 # ||| / delay 3147 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3148 # | | | |||| | | 3149 <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 3150 sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 3151 [...] 3152 <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 3153 3154 # cat trace 3155 # tracer: nop 3156 # 3157 # entries-in-buffer/entries-written: 77/77 #P:8 3158 # 3159 # _-----=> irqs-off 3160 # / _----=> need-resched 3161 # | / _---=> hardirq/softirq 3162 # || / _--=> preempt-depth 3163 # ||| / delay 3164 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3165 # | | | |||| | | 3166 <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 3167 snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 3168 [...] 3169 3170 3171If you try to use this snapshot feature when current tracer is 3172one of the latency tracers, you will get the following results. 3173:: 3174 3175 # echo wakeup > current_tracer 3176 # echo 1 > snapshot 3177 bash: echo: write error: Device or resource busy 3178 # cat snapshot 3179 cat: snapshot: Device or resource busy 3180 3181 3182Instances 3183--------- 3184In the tracefs tracing directory is a directory called "instances". 3185This directory can have new directories created inside of it using 3186mkdir, and removing directories with rmdir. The directory created 3187with mkdir in this directory will already contain files and other 3188directories after it is created. 3189:: 3190 3191 # mkdir instances/foo 3192 # ls instances/foo 3193 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu 3194 set_event snapshot trace trace_clock trace_marker trace_options 3195 trace_pipe tracing_on 3196 3197As you can see, the new directory looks similar to the tracing directory 3198itself. In fact, it is very similar, except that the buffer and 3199events are agnostic from the main director, or from any other 3200instances that are created. 3201 3202The files in the new directory work just like the files with the 3203same name in the tracing directory except the buffer that is used 3204is a separate and new buffer. The files affect that buffer but do not 3205affect the main buffer with the exception of trace_options. Currently, 3206the trace_options affect all instances and the top level buffer 3207the same, but this may change in future releases. That is, options 3208may become specific to the instance they reside in. 3209 3210Notice that none of the function tracer files are there, nor is 3211current_tracer and available_tracers. This is because the buffers 3212can currently only have events enabled for them. 3213:: 3214 3215 # mkdir instances/foo 3216 # mkdir instances/bar 3217 # mkdir instances/zoot 3218 # echo 100000 > buffer_size_kb 3219 # echo 1000 > instances/foo/buffer_size_kb 3220 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb 3221 # echo function > current_trace 3222 # echo 1 > instances/foo/events/sched/sched_wakeup/enable 3223 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable 3224 # echo 1 > instances/foo/events/sched/sched_switch/enable 3225 # echo 1 > instances/bar/events/irq/enable 3226 # echo 1 > instances/zoot/events/syscalls/enable 3227 # cat trace_pipe 3228 CPU:2 [LOST 11745 EVENTS] 3229 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist 3230 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave 3231 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist 3232 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist 3233 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock 3234 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype 3235 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist 3236 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist 3237 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3238 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics 3239 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process 3240 [...] 3241 3242 # cat instances/foo/trace_pipe 3243 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3244 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3245 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003 3246 <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120 3247 rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120 3248 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000 3249 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000 3250 bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120 3251 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001 3252 kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120 3253 [...] 3254 3255 # cat instances/bar/trace_pipe 3256 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX] 3257 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX] 3258 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER] 3259 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU] 3260 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER] 3261 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER] 3262 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU] 3263 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU] 3264 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4 3265 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled 3266 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0 3267 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled 3268 [...] 3269 3270 # cat instances/zoot/trace 3271 # tracer: nop 3272 # 3273 # entries-in-buffer/entries-written: 18996/18996 #P:4 3274 # 3275 # _-----=> irqs-off 3276 # / _----=> need-resched 3277 # | / _---=> hardirq/softirq 3278 # || / _--=> preempt-depth 3279 # ||| / delay 3280 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 3281 # | | | |||| | | 3282 bash-1998 [000] d... 140.733501: sys_write -> 0x2 3283 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1) 3284 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1 3285 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0) 3286 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1 3287 bash-1998 [000] d... 140.733510: sys_close(fd: a) 3288 bash-1998 [000] d... 140.733510: sys_close -> 0x0 3289 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8) 3290 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0 3291 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8) 3292 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0 3293 3294You can see that the trace of the top most trace buffer shows only 3295the function tracing. The foo instance displays wakeups and task 3296switches. 3297 3298To remove the instances, simply delete their directories: 3299:: 3300 3301 # rmdir instances/foo 3302 # rmdir instances/bar 3303 # rmdir instances/zoot 3304 3305Note, if a process has a trace file open in one of the instance 3306directories, the rmdir will fail with EBUSY. 3307 3308 3309Stack trace 3310----------- 3311Since the kernel has a fixed sized stack, it is important not to 3312waste it in functions. A kernel developer must be conscience of 3313what they allocate on the stack. If they add too much, the system 3314can be in danger of a stack overflow, and corruption will occur, 3315usually leading to a system panic. 3316 3317There are some tools that check this, usually with interrupts 3318periodically checking usage. But if you can perform a check 3319at every function call that will become very useful. As ftrace provides 3320a function tracer, it makes it convenient to check the stack size 3321at every function call. This is enabled via the stack tracer. 3322 3323CONFIG_STACK_TRACER enables the ftrace stack tracing functionality. 3324To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled. 3325:: 3326 3327 # echo 1 > /proc/sys/kernel/stack_tracer_enabled 3328 3329You can also enable it from the kernel command line to trace 3330the stack size of the kernel during boot up, by adding "stacktrace" 3331to the kernel command line parameter. 3332 3333After running it for a few minutes, the output looks like: 3334:: 3335 3336 # cat stack_max_size 3337 2928 3338 3339 # cat stack_trace 3340 Depth Size Location (18 entries) 3341 ----- ---- -------- 3342 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac 3343 1) 2704 160 find_busiest_group+0x31/0x1f1 3344 2) 2544 256 load_balance+0xd9/0x662 3345 3) 2288 80 idle_balance+0xbb/0x130 3346 4) 2208 128 __schedule+0x26e/0x5b9 3347 5) 2080 16 schedule+0x64/0x66 3348 6) 2064 128 schedule_timeout+0x34/0xe0 3349 7) 1936 112 wait_for_common+0x97/0xf1 3350 8) 1824 16 wait_for_completion+0x1d/0x1f 3351 9) 1808 128 flush_work+0xfe/0x119 3352 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20 3353 11) 1664 48 input_available_p+0x1d/0x5c 3354 12) 1616 48 n_tty_poll+0x6d/0x134 3355 13) 1568 64 tty_poll+0x64/0x7f 3356 14) 1504 880 do_select+0x31e/0x511 3357 15) 624 400 core_sys_select+0x177/0x216 3358 16) 224 96 sys_select+0x91/0xb9 3359 17) 128 128 system_call_fastpath+0x16/0x1b 3360 3361Note, if -mfentry is being used by gcc, functions get traced before 3362they set up the stack frame. This means that leaf level functions 3363are not tested by the stack tracer when -mfentry is used. 3364 3365Currently, -mfentry is used by gcc 4.6.0 and above on x86 only. 3366 3367More 3368---- 3369More details can be found in the source code, in the `kernel/trace/*.c` files. 3370