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diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst new file mode 100644 index 000000000..c9ab6af4d --- /dev/null +++ b/Documentation/RCU/stallwarn.rst @@ -0,0 +1,336 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================== +Using RCU's CPU Stall Detector +============================== + +This document first discusses what sorts of issues RCU's CPU stall +detector can locate, and then discusses kernel parameters and Kconfig +options that can be used to fine-tune the detector's operation. Finally, +this document explains the stall detector's "splat" format. + + +What Causes RCU CPU Stall Warnings? +=================================== + +So your kernel printed an RCU CPU stall warning. The next question is +"What caused it?" The following problems can result in RCU CPU stall +warnings: + +- A CPU looping in an RCU read-side critical section. + +- A CPU looping with interrupts disabled. + +- A CPU looping with preemption disabled. + +- A CPU looping with bottom halves disabled. + +- For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel + without invoking schedule(). If the looping in the kernel is + really expected and desirable behavior, you might need to add + some calls to cond_resched(). + +- Booting Linux using a console connection that is too slow to + keep up with the boot-time console-message rate. For example, + a 115Kbaud serial console can be -way- too slow to keep up + with boot-time message rates, and will frequently result in + RCU CPU stall warning messages. Especially if you have added + debug printk()s. + +- Anything that prevents RCU's grace-period kthreads from running. + This can result in the "All QSes seen" console-log message. + This message will include information on when the kthread last + ran and how often it should be expected to run. It can also + result in the ``rcu_.*kthread starved for`` console-log message, + which will include additional debugging information. + +- A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might + happen to preempt a low-priority task in the middle of an RCU + read-side critical section. This is especially damaging if + that low-priority task is not permitted to run on any other CPU, + in which case the next RCU grace period can never complete, which + will eventually cause the system to run out of memory and hang. + While the system is in the process of running itself out of + memory, you might see stall-warning messages. + +- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that + is running at a higher priority than the RCU softirq threads. + This will prevent RCU callbacks from ever being invoked, + and in a CONFIG_PREEMPT_RCU kernel will further prevent + RCU grace periods from ever completing. Either way, the + system will eventually run out of memory and hang. In the + CONFIG_PREEMPT_RCU case, you might see stall-warning + messages. + + You can use the rcutree.kthread_prio kernel boot parameter to + increase the scheduling priority of RCU's kthreads, which can + help avoid this problem. However, please note that doing this + can increase your system's context-switch rate and thus degrade + performance. + +- A periodic interrupt whose handler takes longer than the time + interval between successive pairs of interrupts. This can + prevent RCU's kthreads and softirq handlers from running. + Note that certain high-overhead debugging options, for example + the function_graph tracer, can result in interrupt handler taking + considerably longer than normal, which can in turn result in + RCU CPU stall warnings. + +- Testing a workload on a fast system, tuning the stall-warning + timeout down to just barely avoid RCU CPU stall warnings, and then + running the same workload with the same stall-warning timeout on a + slow system. Note that thermal throttling and on-demand governors + can cause a single system to be sometimes fast and sometimes slow! + +- A hardware or software issue shuts off the scheduler-clock + interrupt on a CPU that is not in dyntick-idle mode. This + problem really has happened, and seems to be most likely to + result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. + +- A hardware or software issue that prevents time-based wakeups + from occurring. These issues can range from misconfigured or + buggy timer hardware through bugs in the interrupt or exception + path (whether hardware, firmware, or software) through bugs + in Linux's timer subsystem through bugs in the scheduler, and, + yes, even including bugs in RCU itself. + +- A bug in the RCU implementation. + +- A hardware failure. This is quite unlikely, but has occurred + at least once in real life. A CPU failed in a running system, + becoming unresponsive, but not causing an immediate crash. + This resulted in a series of RCU CPU stall warnings, eventually + leading the realization that the CPU had failed. + +The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. +Note that SRCU does -not- have CPU stall warnings. Please note that +RCU only detects CPU stalls when there is a grace period in progress. +No grace period, no CPU stall warnings. + +To diagnose the cause of the stall, inspect the stack traces. +The offending function will usually be near the top of the stack. +If you have a series of stall warnings from a single extended stall, +comparing the stack traces can often help determine where the stall +is occurring, which will usually be in the function nearest the top of +that portion of the stack which remains the same from trace to trace. +If you can reliably trigger the stall, ftrace can be quite helpful. + +RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE +and with RCU's event tracing. For information on RCU's event tracing, +see include/trace/events/rcu.h. + + +Fine-Tuning the RCU CPU Stall Detector +====================================== + +The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's +CPU stall detector, which detects conditions that unduly delay RCU grace +periods. This module parameter enables CPU stall detection by default, +but may be overridden via boot-time parameter or at runtime via sysfs. +The stall detector's idea of what constitutes "unduly delayed" is +controlled by a set of kernel configuration variables and cpp macros: + +CONFIG_RCU_CPU_STALL_TIMEOUT +---------------------------- + + This kernel configuration parameter defines the period of time + that RCU will wait from the beginning of a grace period until it + issues an RCU CPU stall warning. This time period is normally + 21 seconds. + + This configuration parameter may be changed at runtime via the + /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however + this parameter is checked only at the beginning of a cycle. + So if you are 10 seconds into a 40-second stall, setting this + sysfs parameter to (say) five will shorten the timeout for the + -next- stall, or the following warning for the current stall + (assuming the stall lasts long enough). It will not affect the + timing of the next warning for the current stall. + + Stall-warning messages may be enabled and disabled completely via + /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. + +RCU_STALL_DELAY_DELTA +--------------------- + + Although the lockdep facility is extremely useful, it does add + some overhead. Therefore, under CONFIG_PROVE_RCU, the + RCU_STALL_DELAY_DELTA macro allows five extra seconds before + giving an RCU CPU stall warning message. (This is a cpp + macro, not a kernel configuration parameter.) + +RCU_STALL_RAT_DELAY +------------------- + + The CPU stall detector tries to make the offending CPU print its + own warnings, as this often gives better-quality stack traces. + However, if the offending CPU does not detect its own stall in + the number of jiffies specified by RCU_STALL_RAT_DELAY, then + some other CPU will complain. This delay is normally set to + two jiffies. (This is a cpp macro, not a kernel configuration + parameter.) + +rcupdate.rcu_task_stall_timeout +------------------------------- + + This boot/sysfs parameter controls the RCU-tasks stall warning + interval. A value of zero or less suppresses RCU-tasks stall + warnings. A positive value sets the stall-warning interval + in seconds. An RCU-tasks stall warning starts with the line: + + INFO: rcu_tasks detected stalls on tasks: + + And continues with the output of sched_show_task() for each + task stalling the current RCU-tasks grace period. + + +Interpreting RCU's CPU Stall-Detector "Splats" +============================================== + +For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling, +it will print a message similar to the following:: + + INFO: rcu_sched detected stalls on CPUs/tasks: + 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 + 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0 + (detected by 32, t=2603 jiffies, g=7075, q=625) + +This message indicates that CPU 32 detected that CPUs 2 and 16 were both +causing stalls, and that the stall was affecting RCU-sched. This message +will normally be followed by stack dumps for each CPU. Please note that +PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that +the tasks will be indicated by PID, for example, "P3421". It is even +possible for an rcu_state stall to be caused by both CPUs -and- tasks, +in which case the offending CPUs and tasks will all be called out in the list. + +CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with +the RCU core for the past three grace periods. In contrast, CPU 16's "(0 +ticks this GP)" indicates that this CPU has not taken any scheduling-clock +interrupts during the current stalled grace period. + +The "idle=" portion of the message prints the dyntick-idle state. +The hex number before the first "/" is the low-order 12 bits of the +dynticks counter, which will have an even-numbered value if the CPU +is in dyntick-idle mode and an odd-numbered value otherwise. The hex +number between the two "/"s is the value of the nesting, which will be +a small non-negative number if in the idle loop (as shown above) and a +very large positive number otherwise. + +The "softirq=" portion of the message tracks the number of RCU softirq +handlers that the stalled CPU has executed. The number before the "/" +is the number that had executed since boot at the time that this CPU +last noted the beginning of a grace period, which might be the current +(stalled) grace period, or it might be some earlier grace period (for +example, if the CPU might have been in dyntick-idle mode for an extended +time period. The number after the "/" is the number that have executed +since boot until the current time. If this latter number stays constant +across repeated stall-warning messages, it is possible that RCU's softirq +handlers are no longer able to execute on this CPU. This can happen if +the stalled CPU is spinning with interrupts are disabled, or, in -rt +kernels, if a high-priority process is starving RCU's softirq handler. + +The "fqs=" shows the number of force-quiescent-state idle/offline +detection passes that the grace-period kthread has made across this +CPU since the last time that this CPU noted the beginning of a grace +period. + +The "detected by" line indicates which CPU detected the stall (in this +case, CPU 32), how many jiffies have elapsed since the start of the grace +period (in this case 2603), the grace-period sequence number (7075), and +an estimate of the total number of RCU callbacks queued across all CPUs +(625 in this case). + +In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed +for each CPU:: + + 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1 + +The "last_accelerate:" prints the low-order 16 bits (in hex) of the +jiffies counter when this CPU last invoked rcu_try_advance_all_cbs() +from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from +rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle +processing is enabled. + +If the grace period ends just as the stall warning starts printing, +there will be a spurious stall-warning message, which will include +the following:: + + INFO: Stall ended before state dump start + +This is rare, but does happen from time to time in real life. It is also +possible for a zero-jiffy stall to be flagged in this case, depending +on how the stall warning and the grace-period initialization happen to +interact. Please note that it is not possible to entirely eliminate this +sort of false positive without resorting to things like stop_machine(), +which is overkill for this sort of problem. + +If all CPUs and tasks have passed through quiescent states, but the +grace period has nevertheless failed to end, the stall-warning splat +will include something like the following:: + + All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 + +The "23807" indicates that it has been more than 23 thousand jiffies +since the grace-period kthread ran. The "jiffies_till_next_fqs" +indicates how frequently that kthread should run, giving the number +of jiffies between force-quiescent-state scans, in this case three, +which is way less than 23807. Finally, the root rcu_node structure's +->qsmask field is printed, which will normally be zero. + +If the relevant grace-period kthread has been unable to run prior to +the stall warning, as was the case in the "All QSes seen" line above, +the following additional line is printed:: + + kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 + +Starving the grace-period kthreads of CPU time can of course result +in RCU CPU stall warnings even when all CPUs and tasks have passed +through the required quiescent states. The "g" number shows the current +grace-period sequence number, the "f" precedes the ->gp_flags command +to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the +kthread is waiting for a short timeout, the "state" precedes value of the +task_struct ->state field, and the "cpu" indicates that the grace-period +kthread last ran on CPU 5. + + +Multiple Warnings From One Stall +================================ + +If a stall lasts long enough, multiple stall-warning messages will be +printed for it. The second and subsequent messages are printed at +longer intervals, so that the time between (say) the first and second +message will be about three times the interval between the beginning +of the stall and the first message. + + +Stall Warnings for Expedited Grace Periods +========================================== + +If an expedited grace period detects a stall, it will place a message +like the following in dmesg:: + + INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. + +This indicates that CPU 7 has failed to respond to a reschedule IPI. +The three periods (".") following the CPU number indicate that the CPU +is online (otherwise the first period would instead have been "O"), +that the CPU was online at the beginning of the expedited grace period +(otherwise the second period would have instead been "o"), and that +the CPU has been online at least once since boot (otherwise, the third +period would instead have been "N"). The number before the "jiffies" +indicates that the expedited grace period has been going on for 21,119 +jiffies. The number following the "s:" indicates that the expedited +grace-period sequence counter is 73. The fact that this last value is +odd indicates that an expedited grace period is in flight. The number +following "root:" is a bitmask that indicates which children of the root +rcu_node structure correspond to CPUs and/or tasks that are blocking the +current expedited grace period. If the tree had more than one level, +additional hex numbers would be printed for the states of the other +rcu_node structures in the tree. + +As with normal grace periods, PREEMPT_RCU builds can be stalled by +tasks as well as by CPUs, and that the tasks will be indicated by PID, +for example, "P3421". + +It is entirely possible to see stall warnings from normal and from +expedited grace periods at about the same time during the same run. |