1.. SPDX-License-Identifier: GPL-2.0 2 3============================== 4Using RCU's CPU Stall Detector 5============================== 6 7This document first discusses what sorts of issues RCU's CPU stall 8detector can locate, and then discusses kernel parameters and Kconfig 9options that can be used to fine-tune the detector's operation. Finally, 10this document explains the stall detector's "splat" format. 11 12 13What Causes RCU CPU Stall Warnings? 14=================================== 15 16So your kernel printed an RCU CPU stall warning. The next question is 17"What caused it?" The following problems can result in RCU CPU stall 18warnings: 19 20- A CPU looping in an RCU read-side critical section. 21 22- A CPU looping with interrupts disabled. 23 24- A CPU looping with preemption disabled. 25 26- A CPU looping with bottom halves disabled. 27 28- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel 29 without invoking schedule(). If the looping in the kernel is 30 really expected and desirable behavior, you might need to add 31 some calls to cond_resched(). 32 33- Booting Linux using a console connection that is too slow to 34 keep up with the boot-time console-message rate. For example, 35 a 115Kbaud serial console can be *way* too slow to keep up 36 with boot-time message rates, and will frequently result in 37 RCU CPU stall warning messages. Especially if you have added 38 debug printk()s. 39 40- Anything that prevents RCU's grace-period kthreads from running. 41 This can result in the "All QSes seen" console-log message. 42 This message will include information on when the kthread last 43 ran and how often it should be expected to run. It can also 44 result in the ``rcu_.*kthread starved for`` console-log message, 45 which will include additional debugging information. 46 47- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might 48 happen to preempt a low-priority task in the middle of an RCU 49 read-side critical section. This is especially damaging if 50 that low-priority task is not permitted to run on any other CPU, 51 in which case the next RCU grace period can never complete, which 52 will eventually cause the system to run out of memory and hang. 53 While the system is in the process of running itself out of 54 memory, you might see stall-warning messages. 55 56- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that 57 is running at a higher priority than the RCU softirq threads. 58 This will prevent RCU callbacks from ever being invoked, 59 and in a CONFIG_PREEMPT_RCU kernel will further prevent 60 RCU grace periods from ever completing. Either way, the 61 system will eventually run out of memory and hang. In the 62 CONFIG_PREEMPT_RCU case, you might see stall-warning 63 messages. 64 65 You can use the rcutree.kthread_prio kernel boot parameter to 66 increase the scheduling priority of RCU's kthreads, which can 67 help avoid this problem. However, please note that doing this 68 can increase your system's context-switch rate and thus degrade 69 performance. 70 71- A periodic interrupt whose handler takes longer than the time 72 interval between successive pairs of interrupts. This can 73 prevent RCU's kthreads and softirq handlers from running. 74 Note that certain high-overhead debugging options, for example 75 the function_graph tracer, can result in interrupt handler taking 76 considerably longer than normal, which can in turn result in 77 RCU CPU stall warnings. 78 79- Testing a workload on a fast system, tuning the stall-warning 80 timeout down to just barely avoid RCU CPU stall warnings, and then 81 running the same workload with the same stall-warning timeout on a 82 slow system. Note that thermal throttling and on-demand governors 83 can cause a single system to be sometimes fast and sometimes slow! 84 85- A hardware or software issue shuts off the scheduler-clock 86 interrupt on a CPU that is not in dyntick-idle mode. This 87 problem really has happened, and seems to be most likely to 88 result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. 89 90- A hardware or software issue that prevents time-based wakeups 91 from occurring. These issues can range from misconfigured or 92 buggy timer hardware through bugs in the interrupt or exception 93 path (whether hardware, firmware, or software) through bugs 94 in Linux's timer subsystem through bugs in the scheduler, and, 95 yes, even including bugs in RCU itself. It can also result in 96 the ``rcu_.*timer wakeup didn't happen for`` console-log message, 97 which will include additional debugging information. 98 99- A bug in the RCU implementation. 100 101- A hardware failure. This is quite unlikely, but has occurred 102 at least once in real life. A CPU failed in a running system, 103 becoming unresponsive, but not causing an immediate crash. 104 This resulted in a series of RCU CPU stall warnings, eventually 105 leading the realization that the CPU had failed. 106 107The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. 108Note that SRCU does *not* have CPU stall warnings. Please note that 109RCU only detects CPU stalls when there is a grace period in progress. 110No grace period, no CPU stall warnings. 111 112To diagnose the cause of the stall, inspect the stack traces. 113The offending function will usually be near the top of the stack. 114If you have a series of stall warnings from a single extended stall, 115comparing the stack traces can often help determine where the stall 116is occurring, which will usually be in the function nearest the top of 117that portion of the stack which remains the same from trace to trace. 118If you can reliably trigger the stall, ftrace can be quite helpful. 119 120RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE 121and with RCU's event tracing. For information on RCU's event tracing, 122see include/trace/events/rcu.h. 123 124 125Fine-Tuning the RCU CPU Stall Detector 126====================================== 127 128The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's 129CPU stall detector, which detects conditions that unduly delay RCU grace 130periods. This module parameter enables CPU stall detection by default, 131but may be overridden via boot-time parameter or at runtime via sysfs. 132The stall detector's idea of what constitutes "unduly delayed" is 133controlled by a set of kernel configuration variables and cpp macros: 134 135CONFIG_RCU_CPU_STALL_TIMEOUT 136---------------------------- 137 138 This kernel configuration parameter defines the period of time 139 that RCU will wait from the beginning of a grace period until it 140 issues an RCU CPU stall warning. This time period is normally 141 21 seconds. 142 143 This configuration parameter may be changed at runtime via the 144 /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however 145 this parameter is checked only at the beginning of a cycle. 146 So if you are 10 seconds into a 40-second stall, setting this 147 sysfs parameter to (say) five will shorten the timeout for the 148 *next* stall, or the following warning for the current stall 149 (assuming the stall lasts long enough). It will not affect the 150 timing of the next warning for the current stall. 151 152 Stall-warning messages may be enabled and disabled completely via 153 /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. 154 155RCU_STALL_DELAY_DELTA 156--------------------- 157 158 Although the lockdep facility is extremely useful, it does add 159 some overhead. Therefore, under CONFIG_PROVE_RCU, the 160 RCU_STALL_DELAY_DELTA macro allows five extra seconds before 161 giving an RCU CPU stall warning message. (This is a cpp 162 macro, not a kernel configuration parameter.) 163 164RCU_STALL_RAT_DELAY 165------------------- 166 167 The CPU stall detector tries to make the offending CPU print its 168 own warnings, as this often gives better-quality stack traces. 169 However, if the offending CPU does not detect its own stall in 170 the number of jiffies specified by RCU_STALL_RAT_DELAY, then 171 some other CPU will complain. This delay is normally set to 172 two jiffies. (This is a cpp macro, not a kernel configuration 173 parameter.) 174 175rcupdate.rcu_task_stall_timeout 176------------------------------- 177 178 This boot/sysfs parameter controls the RCU-tasks stall warning 179 interval. A value of zero or less suppresses RCU-tasks stall 180 warnings. A positive value sets the stall-warning interval 181 in seconds. An RCU-tasks stall warning starts with the line: 182 183 INFO: rcu_tasks detected stalls on tasks: 184 185 And continues with the output of sched_show_task() for each 186 task stalling the current RCU-tasks grace period. 187 188 189Interpreting RCU's CPU Stall-Detector "Splats" 190============================================== 191 192For non-RCU-tasks flavors of RCU, when a CPU detects that some other 193CPU is stalling, it will print a message similar to the following:: 194 195 INFO: rcu_sched detected stalls on CPUs/tasks: 196 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 197 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0 198 (detected by 32, t=2603 jiffies, g=7075, q=625) 199 200This message indicates that CPU 32 detected that CPUs 2 and 16 were both 201causing stalls, and that the stall was affecting RCU-sched. This message 202will normally be followed by stack dumps for each CPU. Please note that 203PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that 204the tasks will be indicated by PID, for example, "P3421". It is even 205possible for an rcu_state stall to be caused by both CPUs *and* tasks, 206in which case the offending CPUs and tasks will all be called out in the list. 207In some cases, CPUs will detect themselves stalling, which will result 208in a self-detected stall. 209 210CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with 211the RCU core for the past three grace periods. In contrast, CPU 16's "(0 212ticks this GP)" indicates that this CPU has not taken any scheduling-clock 213interrupts during the current stalled grace period. 214 215The "idle=" portion of the message prints the dyntick-idle state. 216The hex number before the first "/" is the low-order 12 bits of the 217dynticks counter, which will have an even-numbered value if the CPU 218is in dyntick-idle mode and an odd-numbered value otherwise. The hex 219number between the two "/"s is the value of the nesting, which will be 220a small non-negative number if in the idle loop (as shown above) and a 221very large positive number otherwise. 222 223The "softirq=" portion of the message tracks the number of RCU softirq 224handlers that the stalled CPU has executed. The number before the "/" 225is the number that had executed since boot at the time that this CPU 226last noted the beginning of a grace period, which might be the current 227(stalled) grace period, or it might be some earlier grace period (for 228example, if the CPU might have been in dyntick-idle mode for an extended 229time period). The number after the "/" is the number that have executed 230since boot until the current time. If this latter number stays constant 231across repeated stall-warning messages, it is possible that RCU's softirq 232handlers are no longer able to execute on this CPU. This can happen if 233the stalled CPU is spinning with interrupts are disabled, or, in -rt 234kernels, if a high-priority process is starving RCU's softirq handler. 235 236The "fqs=" shows the number of force-quiescent-state idle/offline 237detection passes that the grace-period kthread has made across this 238CPU since the last time that this CPU noted the beginning of a grace 239period. 240 241The "detected by" line indicates which CPU detected the stall (in this 242case, CPU 32), how many jiffies have elapsed since the start of the grace 243period (in this case 2603), the grace-period sequence number (7075), and 244an estimate of the total number of RCU callbacks queued across all CPUs 245(625 in this case). 246 247In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed 248for each CPU:: 249 250 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1 251 252The "last_accelerate:" prints the low-order 16 bits (in hex) of the 253jiffies counter when this CPU last invoked rcu_try_advance_all_cbs() 254from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from 255rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle 256processing is enabled. 257 258If the grace period ends just as the stall warning starts printing, 259there will be a spurious stall-warning message, which will include 260the following:: 261 262 INFO: Stall ended before state dump start 263 264This is rare, but does happen from time to time in real life. It is also 265possible for a zero-jiffy stall to be flagged in this case, depending 266on how the stall warning and the grace-period initialization happen to 267interact. Please note that it is not possible to entirely eliminate this 268sort of false positive without resorting to things like stop_machine(), 269which is overkill for this sort of problem. 270 271If all CPUs and tasks have passed through quiescent states, but the 272grace period has nevertheless failed to end, the stall-warning splat 273will include something like the following:: 274 275 All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 276 277The "23807" indicates that it has been more than 23 thousand jiffies 278since the grace-period kthread ran. The "jiffies_till_next_fqs" 279indicates how frequently that kthread should run, giving the number 280of jiffies between force-quiescent-state scans, in this case three, 281which is way less than 23807. Finally, the root rcu_node structure's 282->qsmask field is printed, which will normally be zero. 283 284If the relevant grace-period kthread has been unable to run prior to 285the stall warning, as was the case in the "All QSes seen" line above, 286the following additional line is printed:: 287 288 rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 289 Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. 290 291Starving the grace-period kthreads of CPU time can of course result 292in RCU CPU stall warnings even when all CPUs and tasks have passed 293through the required quiescent states. The "g" number shows the current 294grace-period sequence number, the "f" precedes the ->gp_flags command 295to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the 296kthread is waiting for a short timeout, the "state" precedes value of the 297task_struct ->state field, and the "cpu" indicates that the grace-period 298kthread last ran on CPU 5. 299 300If the relevant grace-period kthread does not wake from FQS wait in a 301reasonable time, then the following additional line is printed:: 302 303 kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 304 305The "23804" indicates that kthread's timer expired more than 23 thousand 306jiffies ago. The rest of the line has meaning similar to the kthread 307starvation case. 308 309Additionally, the following line is printed:: 310 311 Possible timer handling issue on cpu=4 timer-softirq=11142 312 313Here "cpu" indicates that the grace-period kthread last ran on CPU 4, 314where it queued the fqs timer. The number following the "timer-softirq" 315is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not 316change on successive RCU CPU stall warnings, there is further reason to 317suspect a timer problem. 318 319These messages are usually followed by stack dumps of the CPUs and tasks 320involved in the stall. These stack traces can help you locate the cause 321of the stall, keeping in mind that the CPU detecting the stall will have 322an interrupt frame that is mainly devoted to detecting the stall. 323 324 325Multiple Warnings From One Stall 326================================ 327 328If a stall lasts long enough, multiple stall-warning messages will 329be printed for it. The second and subsequent messages are printed at 330longer intervals, so that the time between (say) the first and second 331message will be about three times the interval between the beginning 332of the stall and the first message. It can be helpful to compare the 333stack dumps for the different messages for the same stalled grace period. 334 335 336Stall Warnings for Expedited Grace Periods 337========================================== 338 339If an expedited grace period detects a stall, it will place a message 340like the following in dmesg:: 341 342 INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. 343 344This indicates that CPU 7 has failed to respond to a reschedule IPI. 345The three periods (".") following the CPU number indicate that the CPU 346is online (otherwise the first period would instead have been "O"), 347that the CPU was online at the beginning of the expedited grace period 348(otherwise the second period would have instead been "o"), and that 349the CPU has been online at least once since boot (otherwise, the third 350period would instead have been "N"). The number before the "jiffies" 351indicates that the expedited grace period has been going on for 21,119 352jiffies. The number following the "s:" indicates that the expedited 353grace-period sequence counter is 73. The fact that this last value is 354odd indicates that an expedited grace period is in flight. The number 355following "root:" is a bitmask that indicates which children of the root 356rcu_node structure correspond to CPUs and/or tasks that are blocking the 357current expedited grace period. If the tree had more than one level, 358additional hex numbers would be printed for the states of the other 359rcu_node structures in the tree. 360 361As with normal grace periods, PREEMPT_RCU builds can be stalled by 362tasks as well as by CPUs, and that the tasks will be indicated by PID, 363for example, "P3421". 364 365It is entirely possible to see stall warnings from normal and from 366expedited grace periods at about the same time during the same run. 367