1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * trace_hwlat.c - A simple Hardware Latency detector. 4 * 5 * Use this tracer to detect large system latencies induced by the behavior of 6 * certain underlying system hardware or firmware, independent of Linux itself. 7 * The code was developed originally to detect the presence of SMIs on Intel 8 * and AMD systems, although there is no dependency upon x86 herein. 9 * 10 * The classical example usage of this tracer is in detecting the presence of 11 * SMIs or System Management Interrupts on Intel and AMD systems. An SMI is a 12 * somewhat special form of hardware interrupt spawned from earlier CPU debug 13 * modes in which the (BIOS/EFI/etc.) firmware arranges for the South Bridge 14 * LPC (or other device) to generate a special interrupt under certain 15 * circumstances, for example, upon expiration of a special SMI timer device, 16 * due to certain external thermal readings, on certain I/O address accesses, 17 * and other situations. An SMI hits a special CPU pin, triggers a special 18 * SMI mode (complete with special memory map), and the OS is unaware. 19 * 20 * Although certain hardware-inducing latencies are necessary (for example, 21 * a modern system often requires an SMI handler for correct thermal control 22 * and remote management) they can wreak havoc upon any OS-level performance 23 * guarantees toward low-latency, especially when the OS is not even made 24 * aware of the presence of these interrupts. For this reason, we need a 25 * somewhat brute force mechanism to detect these interrupts. In this case, 26 * we do it by hogging all of the CPU(s) for configurable timer intervals, 27 * sampling the built-in CPU timer, looking for discontiguous readings. 28 * 29 * WARNING: This implementation necessarily introduces latencies. Therefore, 30 * you should NEVER use this tracer while running in a production 31 * environment requiring any kind of low-latency performance 32 * guarantee(s). 33 * 34 * Copyright (C) 2008-2009 Jon Masters, Red Hat, Inc. <jcm@redhat.com> 35 * Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <srostedt@redhat.com> 36 * 37 * Includes useful feedback from Clark Williams <clark@redhat.com> 38 * 39 */ 40 #include <linux/kthread.h> 41 #include <linux/tracefs.h> 42 #include <linux/uaccess.h> 43 #include <linux/cpumask.h> 44 #include <linux/delay.h> 45 #include <linux/sched/clock.h> 46 #include "trace.h" 47 48 static struct trace_array *hwlat_trace; 49 50 #define U64STR_SIZE 22 /* 20 digits max */ 51 52 #define BANNER "hwlat_detector: " 53 #define DEFAULT_SAMPLE_WINDOW 1000000 /* 1s */ 54 #define DEFAULT_SAMPLE_WIDTH 500000 /* 0.5s */ 55 #define DEFAULT_LAT_THRESHOLD 10 /* 10us */ 56 57 /* sampling thread*/ 58 static struct task_struct *hwlat_kthread; 59 60 static struct dentry *hwlat_sample_width; /* sample width us */ 61 static struct dentry *hwlat_sample_window; /* sample window us */ 62 63 /* Save the previous tracing_thresh value */ 64 static unsigned long save_tracing_thresh; 65 66 /* NMI timestamp counters */ 67 static u64 nmi_ts_start; 68 static u64 nmi_total_ts; 69 static int nmi_count; 70 static int nmi_cpu; 71 72 /* Tells NMIs to call back to the hwlat tracer to record timestamps */ 73 bool trace_hwlat_callback_enabled; 74 75 /* If the user changed threshold, remember it */ 76 static u64 last_tracing_thresh = DEFAULT_LAT_THRESHOLD * NSEC_PER_USEC; 77 78 /* Individual latency samples are stored here when detected. */ 79 struct hwlat_sample { 80 u64 seqnum; /* unique sequence */ 81 u64 duration; /* delta */ 82 u64 outer_duration; /* delta (outer loop) */ 83 u64 nmi_total_ts; /* Total time spent in NMIs */ 84 struct timespec64 timestamp; /* wall time */ 85 int nmi_count; /* # NMIs during this sample */ 86 int count; /* # of iteratons over threash */ 87 }; 88 89 /* keep the global state somewhere. */ 90 static struct hwlat_data { 91 92 struct mutex lock; /* protect changes */ 93 94 u64 count; /* total since reset */ 95 96 u64 sample_window; /* total sampling window (on+off) */ 97 u64 sample_width; /* active sampling portion of window */ 98 99 } hwlat_data = { 100 .sample_window = DEFAULT_SAMPLE_WINDOW, 101 .sample_width = DEFAULT_SAMPLE_WIDTH, 102 }; 103 104 static void trace_hwlat_sample(struct hwlat_sample *sample) 105 { 106 struct trace_array *tr = hwlat_trace; 107 struct trace_event_call *call = &event_hwlat; 108 struct trace_buffer *buffer = tr->array_buffer.buffer; 109 struct ring_buffer_event *event; 110 struct hwlat_entry *entry; 111 unsigned long flags; 112 int pc; 113 114 pc = preempt_count(); 115 local_save_flags(flags); 116 117 event = trace_buffer_lock_reserve(buffer, TRACE_HWLAT, sizeof(*entry), 118 flags, pc); 119 if (!event) 120 return; 121 entry = ring_buffer_event_data(event); 122 entry->seqnum = sample->seqnum; 123 entry->duration = sample->duration; 124 entry->outer_duration = sample->outer_duration; 125 entry->timestamp = sample->timestamp; 126 entry->nmi_total_ts = sample->nmi_total_ts; 127 entry->nmi_count = sample->nmi_count; 128 entry->count = sample->count; 129 130 if (!call_filter_check_discard(call, entry, buffer, event)) 131 trace_buffer_unlock_commit_nostack(buffer, event); 132 } 133 134 /* Macros to encapsulate the time capturing infrastructure */ 135 #define time_type u64 136 #define time_get() trace_clock_local() 137 #define time_to_us(x) div_u64(x, 1000) 138 #define time_sub(a, b) ((a) - (b)) 139 #define init_time(a, b) (a = b) 140 #define time_u64(a) a 141 142 void trace_hwlat_callback(bool enter) 143 { 144 if (smp_processor_id() != nmi_cpu) 145 return; 146 147 /* 148 * Currently trace_clock_local() calls sched_clock() and the 149 * generic version is not NMI safe. 150 */ 151 if (!IS_ENABLED(CONFIG_GENERIC_SCHED_CLOCK)) { 152 if (enter) 153 nmi_ts_start = time_get(); 154 else 155 nmi_total_ts += time_get() - nmi_ts_start; 156 } 157 158 if (enter) 159 nmi_count++; 160 } 161 162 /** 163 * get_sample - sample the CPU TSC and look for likely hardware latencies 164 * 165 * Used to repeatedly capture the CPU TSC (or similar), looking for potential 166 * hardware-induced latency. Called with interrupts disabled and with 167 * hwlat_data.lock held. 168 */ 169 static int get_sample(void) 170 { 171 struct trace_array *tr = hwlat_trace; 172 struct hwlat_sample s; 173 time_type start, t1, t2, last_t2; 174 s64 diff, outer_diff, total, last_total = 0; 175 u64 sample = 0; 176 u64 thresh = tracing_thresh; 177 u64 outer_sample = 0; 178 int ret = -1; 179 unsigned int count = 0; 180 181 do_div(thresh, NSEC_PER_USEC); /* modifies interval value */ 182 183 nmi_cpu = smp_processor_id(); 184 nmi_total_ts = 0; 185 nmi_count = 0; 186 /* Make sure NMIs see this first */ 187 barrier(); 188 189 trace_hwlat_callback_enabled = true; 190 191 init_time(last_t2, 0); 192 start = time_get(); /* start timestamp */ 193 outer_diff = 0; 194 195 do { 196 197 t1 = time_get(); /* we'll look for a discontinuity */ 198 t2 = time_get(); 199 200 if (time_u64(last_t2)) { 201 /* Check the delta from outer loop (t2 to next t1) */ 202 outer_diff = time_to_us(time_sub(t1, last_t2)); 203 /* This shouldn't happen */ 204 if (outer_diff < 0) { 205 pr_err(BANNER "time running backwards\n"); 206 goto out; 207 } 208 if (outer_diff > outer_sample) 209 outer_sample = outer_diff; 210 } 211 last_t2 = t2; 212 213 total = time_to_us(time_sub(t2, start)); /* sample width */ 214 215 /* Check for possible overflows */ 216 if (total < last_total) { 217 pr_err("Time total overflowed\n"); 218 break; 219 } 220 last_total = total; 221 222 /* This checks the inner loop (t1 to t2) */ 223 diff = time_to_us(time_sub(t2, t1)); /* current diff */ 224 225 if (diff > thresh || outer_diff > thresh) { 226 if (!count) 227 ktime_get_real_ts64(&s.timestamp); 228 count++; 229 } 230 231 /* This shouldn't happen */ 232 if (diff < 0) { 233 pr_err(BANNER "time running backwards\n"); 234 goto out; 235 } 236 237 if (diff > sample) 238 sample = diff; /* only want highest value */ 239 240 } while (total <= hwlat_data.sample_width); 241 242 barrier(); /* finish the above in the view for NMIs */ 243 trace_hwlat_callback_enabled = false; 244 barrier(); /* Make sure nmi_total_ts is no longer updated */ 245 246 ret = 0; 247 248 /* If we exceed the threshold value, we have found a hardware latency */ 249 if (sample > thresh || outer_sample > thresh) { 250 u64 latency; 251 252 ret = 1; 253 254 /* We read in microseconds */ 255 if (nmi_total_ts) 256 do_div(nmi_total_ts, NSEC_PER_USEC); 257 258 hwlat_data.count++; 259 s.seqnum = hwlat_data.count; 260 s.duration = sample; 261 s.outer_duration = outer_sample; 262 s.nmi_total_ts = nmi_total_ts; 263 s.nmi_count = nmi_count; 264 s.count = count; 265 trace_hwlat_sample(&s); 266 267 latency = max(sample, outer_sample); 268 269 /* Keep a running maximum ever recorded hardware latency */ 270 if (latency > tr->max_latency) { 271 tr->max_latency = latency; 272 latency_fsnotify(tr); 273 } 274 } 275 276 out: 277 return ret; 278 } 279 280 static struct cpumask save_cpumask; 281 static bool disable_migrate; 282 283 static void move_to_next_cpu(void) 284 { 285 struct cpumask *current_mask = &save_cpumask; 286 struct trace_array *tr = hwlat_trace; 287 int next_cpu; 288 289 if (disable_migrate) 290 return; 291 /* 292 * If for some reason the user modifies the CPU affinity 293 * of this thread, then stop migrating for the duration 294 * of the current test. 295 */ 296 if (!cpumask_equal(current_mask, current->cpus_ptr)) 297 goto disable; 298 299 get_online_cpus(); 300 cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask); 301 next_cpu = cpumask_next(smp_processor_id(), current_mask); 302 put_online_cpus(); 303 304 if (next_cpu >= nr_cpu_ids) 305 next_cpu = cpumask_first(current_mask); 306 307 if (next_cpu >= nr_cpu_ids) /* Shouldn't happen! */ 308 goto disable; 309 310 cpumask_clear(current_mask); 311 cpumask_set_cpu(next_cpu, current_mask); 312 313 sched_setaffinity(0, current_mask); 314 return; 315 316 disable: 317 disable_migrate = true; 318 } 319 320 /* 321 * kthread_fn - The CPU time sampling/hardware latency detection kernel thread 322 * 323 * Used to periodically sample the CPU TSC via a call to get_sample. We 324 * disable interrupts, which does (intentionally) introduce latency since we 325 * need to ensure nothing else might be running (and thus preempting). 326 * Obviously this should never be used in production environments. 327 * 328 * Executes one loop interaction on each CPU in tracing_cpumask sysfs file. 329 */ 330 static int kthread_fn(void *data) 331 { 332 u64 interval; 333 334 while (!kthread_should_stop()) { 335 336 move_to_next_cpu(); 337 338 local_irq_disable(); 339 get_sample(); 340 local_irq_enable(); 341 342 mutex_lock(&hwlat_data.lock); 343 interval = hwlat_data.sample_window - hwlat_data.sample_width; 344 mutex_unlock(&hwlat_data.lock); 345 346 do_div(interval, USEC_PER_MSEC); /* modifies interval value */ 347 348 /* Always sleep for at least 1ms */ 349 if (interval < 1) 350 interval = 1; 351 352 if (msleep_interruptible(interval)) 353 break; 354 } 355 356 return 0; 357 } 358 359 /** 360 * start_kthread - Kick off the hardware latency sampling/detector kthread 361 * 362 * This starts the kernel thread that will sit and sample the CPU timestamp 363 * counter (TSC or similar) and look for potential hardware latencies. 364 */ 365 static int start_kthread(struct trace_array *tr) 366 { 367 struct cpumask *current_mask = &save_cpumask; 368 struct task_struct *kthread; 369 int next_cpu; 370 371 if (hwlat_kthread) 372 return 0; 373 374 /* Just pick the first CPU on first iteration */ 375 get_online_cpus(); 376 cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask); 377 put_online_cpus(); 378 next_cpu = cpumask_first(current_mask); 379 380 kthread = kthread_create(kthread_fn, NULL, "hwlatd"); 381 if (IS_ERR(kthread)) { 382 pr_err(BANNER "could not start sampling thread\n"); 383 return -ENOMEM; 384 } 385 386 cpumask_clear(current_mask); 387 cpumask_set_cpu(next_cpu, current_mask); 388 sched_setaffinity(kthread->pid, current_mask); 389 390 hwlat_kthread = kthread; 391 wake_up_process(kthread); 392 393 return 0; 394 } 395 396 /** 397 * stop_kthread - Inform the hardware latency samping/detector kthread to stop 398 * 399 * This kicks the running hardware latency sampling/detector kernel thread and 400 * tells it to stop sampling now. Use this on unload and at system shutdown. 401 */ 402 static void stop_kthread(void) 403 { 404 if (!hwlat_kthread) 405 return; 406 kthread_stop(hwlat_kthread); 407 hwlat_kthread = NULL; 408 } 409 410 /* 411 * hwlat_read - Wrapper read function for reading both window and width 412 * @filp: The active open file structure 413 * @ubuf: The userspace provided buffer to read value into 414 * @cnt: The maximum number of bytes to read 415 * @ppos: The current "file" position 416 * 417 * This function provides a generic read implementation for the global state 418 * "hwlat_data" structure filesystem entries. 419 */ 420 static ssize_t hwlat_read(struct file *filp, char __user *ubuf, 421 size_t cnt, loff_t *ppos) 422 { 423 char buf[U64STR_SIZE]; 424 u64 *entry = filp->private_data; 425 u64 val; 426 int len; 427 428 if (!entry) 429 return -EFAULT; 430 431 if (cnt > sizeof(buf)) 432 cnt = sizeof(buf); 433 434 val = *entry; 435 436 len = snprintf(buf, sizeof(buf), "%llu\n", val); 437 438 return simple_read_from_buffer(ubuf, cnt, ppos, buf, len); 439 } 440 441 /** 442 * hwlat_width_write - Write function for "width" entry 443 * @filp: The active open file structure 444 * @ubuf: The user buffer that contains the value to write 445 * @cnt: The maximum number of bytes to write to "file" 446 * @ppos: The current position in @file 447 * 448 * This function provides a write implementation for the "width" interface 449 * to the hardware latency detector. It can be used to configure 450 * for how many us of the total window us we will actively sample for any 451 * hardware-induced latency periods. Obviously, it is not possible to 452 * sample constantly and have the system respond to a sample reader, or, 453 * worse, without having the system appear to have gone out to lunch. It 454 * is enforced that width is less that the total window size. 455 */ 456 static ssize_t 457 hwlat_width_write(struct file *filp, const char __user *ubuf, 458 size_t cnt, loff_t *ppos) 459 { 460 u64 val; 461 int err; 462 463 err = kstrtoull_from_user(ubuf, cnt, 10, &val); 464 if (err) 465 return err; 466 467 mutex_lock(&hwlat_data.lock); 468 if (val < hwlat_data.sample_window) 469 hwlat_data.sample_width = val; 470 else 471 err = -EINVAL; 472 mutex_unlock(&hwlat_data.lock); 473 474 if (err) 475 return err; 476 477 return cnt; 478 } 479 480 /** 481 * hwlat_window_write - Write function for "window" entry 482 * @filp: The active open file structure 483 * @ubuf: The user buffer that contains the value to write 484 * @cnt: The maximum number of bytes to write to "file" 485 * @ppos: The current position in @file 486 * 487 * This function provides a write implementation for the "window" interface 488 * to the hardware latency detetector. The window is the total time 489 * in us that will be considered one sample period. Conceptually, windows 490 * occur back-to-back and contain a sample width period during which 491 * actual sampling occurs. Can be used to write a new total window size. It 492 * is enfoced that any value written must be greater than the sample width 493 * size, or an error results. 494 */ 495 static ssize_t 496 hwlat_window_write(struct file *filp, const char __user *ubuf, 497 size_t cnt, loff_t *ppos) 498 { 499 u64 val; 500 int err; 501 502 err = kstrtoull_from_user(ubuf, cnt, 10, &val); 503 if (err) 504 return err; 505 506 mutex_lock(&hwlat_data.lock); 507 if (hwlat_data.sample_width < val) 508 hwlat_data.sample_window = val; 509 else 510 err = -EINVAL; 511 mutex_unlock(&hwlat_data.lock); 512 513 if (err) 514 return err; 515 516 return cnt; 517 } 518 519 static const struct file_operations width_fops = { 520 .open = tracing_open_generic, 521 .read = hwlat_read, 522 .write = hwlat_width_write, 523 }; 524 525 static const struct file_operations window_fops = { 526 .open = tracing_open_generic, 527 .read = hwlat_read, 528 .write = hwlat_window_write, 529 }; 530 531 /** 532 * init_tracefs - A function to initialize the tracefs interface files 533 * 534 * This function creates entries in tracefs for "hwlat_detector". 535 * It creates the hwlat_detector directory in the tracing directory, 536 * and within that directory is the count, width and window files to 537 * change and view those values. 538 */ 539 static int init_tracefs(void) 540 { 541 int ret; 542 struct dentry *top_dir; 543 544 ret = tracing_init_dentry(); 545 if (ret) 546 return -ENOMEM; 547 548 top_dir = tracefs_create_dir("hwlat_detector", NULL); 549 if (!top_dir) 550 return -ENOMEM; 551 552 hwlat_sample_window = tracefs_create_file("window", 0640, 553 top_dir, 554 &hwlat_data.sample_window, 555 &window_fops); 556 if (!hwlat_sample_window) 557 goto err; 558 559 hwlat_sample_width = tracefs_create_file("width", 0644, 560 top_dir, 561 &hwlat_data.sample_width, 562 &width_fops); 563 if (!hwlat_sample_width) 564 goto err; 565 566 return 0; 567 568 err: 569 tracefs_remove(top_dir); 570 return -ENOMEM; 571 } 572 573 static void hwlat_tracer_start(struct trace_array *tr) 574 { 575 int err; 576 577 err = start_kthread(tr); 578 if (err) 579 pr_err(BANNER "Cannot start hwlat kthread\n"); 580 } 581 582 static void hwlat_tracer_stop(struct trace_array *tr) 583 { 584 stop_kthread(); 585 } 586 587 static bool hwlat_busy; 588 589 static int hwlat_tracer_init(struct trace_array *tr) 590 { 591 /* Only allow one instance to enable this */ 592 if (hwlat_busy) 593 return -EBUSY; 594 595 hwlat_trace = tr; 596 597 disable_migrate = false; 598 hwlat_data.count = 0; 599 tr->max_latency = 0; 600 save_tracing_thresh = tracing_thresh; 601 602 /* tracing_thresh is in nsecs, we speak in usecs */ 603 if (!tracing_thresh) 604 tracing_thresh = last_tracing_thresh; 605 606 if (tracer_tracing_is_on(tr)) 607 hwlat_tracer_start(tr); 608 609 hwlat_busy = true; 610 611 return 0; 612 } 613 614 static void hwlat_tracer_reset(struct trace_array *tr) 615 { 616 stop_kthread(); 617 618 /* the tracing threshold is static between runs */ 619 last_tracing_thresh = tracing_thresh; 620 621 tracing_thresh = save_tracing_thresh; 622 hwlat_busy = false; 623 } 624 625 static struct tracer hwlat_tracer __read_mostly = 626 { 627 .name = "hwlat", 628 .init = hwlat_tracer_init, 629 .reset = hwlat_tracer_reset, 630 .start = hwlat_tracer_start, 631 .stop = hwlat_tracer_stop, 632 .allow_instances = true, 633 }; 634 635 __init static int init_hwlat_tracer(void) 636 { 637 int ret; 638 639 mutex_init(&hwlat_data.lock); 640 641 ret = register_tracer(&hwlat_tracer); 642 if (ret) 643 return ret; 644 645 init_tracefs(); 646 647 return 0; 648 } 649 late_initcall(init_hwlat_tracer); 650