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366 lines
17 KiB
366 lines
17 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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============================== |
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Using RCU's CPU Stall Detector |
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============================== |
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This document first discusses what sorts of issues RCU's CPU stall |
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detector can locate, and then discusses kernel parameters and Kconfig |
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options that can be used to fine-tune the detector's operation. Finally, |
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this document explains the stall detector's "splat" format. |
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What Causes RCU CPU Stall Warnings? |
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=================================== |
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So your kernel printed an RCU CPU stall warning. The next question is |
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"What caused it?" The following problems can result in RCU CPU stall |
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warnings: |
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- A CPU looping in an RCU read-side critical section. |
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- A CPU looping with interrupts disabled. |
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- A CPU looping with preemption disabled. |
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- A CPU looping with bottom halves disabled. |
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- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel |
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without invoking schedule(). If the looping in the kernel is |
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really expected and desirable behavior, you might need to add |
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some calls to cond_resched(). |
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- Booting Linux using a console connection that is too slow to |
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keep up with the boot-time console-message rate. For example, |
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a 115Kbaud serial console can be *way* too slow to keep up |
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with boot-time message rates, and will frequently result in |
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RCU CPU stall warning messages. Especially if you have added |
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debug printk()s. |
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- Anything that prevents RCU's grace-period kthreads from running. |
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This can result in the "All QSes seen" console-log message. |
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This message will include information on when the kthread last |
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ran and how often it should be expected to run. It can also |
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result in the ``rcu_.*kthread starved for`` console-log message, |
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which will include additional debugging information. |
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- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might |
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happen to preempt a low-priority task in the middle of an RCU |
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read-side critical section. This is especially damaging if |
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that low-priority task is not permitted to run on any other CPU, |
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in which case the next RCU grace period can never complete, which |
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will eventually cause the system to run out of memory and hang. |
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While the system is in the process of running itself out of |
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memory, you might see stall-warning messages. |
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- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that |
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is running at a higher priority than the RCU softirq threads. |
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This will prevent RCU callbacks from ever being invoked, |
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and in a CONFIG_PREEMPT_RCU kernel will further prevent |
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RCU grace periods from ever completing. Either way, the |
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system will eventually run out of memory and hang. In the |
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CONFIG_PREEMPT_RCU case, you might see stall-warning |
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messages. |
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You can use the rcutree.kthread_prio kernel boot parameter to |
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increase the scheduling priority of RCU's kthreads, which can |
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help avoid this problem. However, please note that doing this |
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can increase your system's context-switch rate and thus degrade |
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performance. |
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- A periodic interrupt whose handler takes longer than the time |
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interval between successive pairs of interrupts. This can |
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prevent RCU's kthreads and softirq handlers from running. |
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Note that certain high-overhead debugging options, for example |
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the function_graph tracer, can result in interrupt handler taking |
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considerably longer than normal, which can in turn result in |
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RCU CPU stall warnings. |
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- Testing a workload on a fast system, tuning the stall-warning |
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timeout down to just barely avoid RCU CPU stall warnings, and then |
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running the same workload with the same stall-warning timeout on a |
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slow system. Note that thermal throttling and on-demand governors |
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can cause a single system to be sometimes fast and sometimes slow! |
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- A hardware or software issue shuts off the scheduler-clock |
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interrupt on a CPU that is not in dyntick-idle mode. This |
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problem really has happened, and seems to be most likely to |
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result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. |
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- A hardware or software issue that prevents time-based wakeups |
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from occurring. These issues can range from misconfigured or |
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buggy timer hardware through bugs in the interrupt or exception |
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path (whether hardware, firmware, or software) through bugs |
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in Linux's timer subsystem through bugs in the scheduler, and, |
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yes, even including bugs in RCU itself. It can also result in |
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the ``rcu_.*timer wakeup didn't happen for`` console-log message, |
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which will include additional debugging information. |
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- A bug in the RCU implementation. |
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- A hardware failure. This is quite unlikely, but has occurred |
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at least once in real life. A CPU failed in a running system, |
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becoming unresponsive, but not causing an immediate crash. |
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This resulted in a series of RCU CPU stall warnings, eventually |
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leading the realization that the CPU had failed. |
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The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. |
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Note that SRCU does *not* have CPU stall warnings. Please note that |
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RCU only detects CPU stalls when there is a grace period in progress. |
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No grace period, no CPU stall warnings. |
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To diagnose the cause of the stall, inspect the stack traces. |
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The offending function will usually be near the top of the stack. |
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If you have a series of stall warnings from a single extended stall, |
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comparing the stack traces can often help determine where the stall |
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is occurring, which will usually be in the function nearest the top of |
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that portion of the stack which remains the same from trace to trace. |
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If you can reliably trigger the stall, ftrace can be quite helpful. |
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RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE |
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and with RCU's event tracing. For information on RCU's event tracing, |
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see include/trace/events/rcu.h. |
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Fine-Tuning the RCU CPU Stall Detector |
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====================================== |
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The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's |
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CPU stall detector, which detects conditions that unduly delay RCU grace |
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periods. This module parameter enables CPU stall detection by default, |
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but may be overridden via boot-time parameter or at runtime via sysfs. |
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The stall detector's idea of what constitutes "unduly delayed" is |
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controlled by a set of kernel configuration variables and cpp macros: |
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CONFIG_RCU_CPU_STALL_TIMEOUT |
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---------------------------- |
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This kernel configuration parameter defines the period of time |
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that RCU will wait from the beginning of a grace period until it |
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issues an RCU CPU stall warning. This time period is normally |
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21 seconds. |
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This configuration parameter may be changed at runtime via the |
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/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however |
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this parameter is checked only at the beginning of a cycle. |
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So if you are 10 seconds into a 40-second stall, setting this |
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sysfs parameter to (say) five will shorten the timeout for the |
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*next* stall, or the following warning for the current stall |
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(assuming the stall lasts long enough). It will not affect the |
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timing of the next warning for the current stall. |
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Stall-warning messages may be enabled and disabled completely via |
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/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. |
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RCU_STALL_DELAY_DELTA |
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--------------------- |
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Although the lockdep facility is extremely useful, it does add |
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some overhead. Therefore, under CONFIG_PROVE_RCU, the |
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RCU_STALL_DELAY_DELTA macro allows five extra seconds before |
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giving an RCU CPU stall warning message. (This is a cpp |
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macro, not a kernel configuration parameter.) |
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RCU_STALL_RAT_DELAY |
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------------------- |
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The CPU stall detector tries to make the offending CPU print its |
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own warnings, as this often gives better-quality stack traces. |
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However, if the offending CPU does not detect its own stall in |
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the number of jiffies specified by RCU_STALL_RAT_DELAY, then |
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some other CPU will complain. This delay is normally set to |
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two jiffies. (This is a cpp macro, not a kernel configuration |
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parameter.) |
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rcupdate.rcu_task_stall_timeout |
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------------------------------- |
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This boot/sysfs parameter controls the RCU-tasks stall warning |
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interval. A value of zero or less suppresses RCU-tasks stall |
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warnings. A positive value sets the stall-warning interval |
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in seconds. An RCU-tasks stall warning starts with the line: |
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INFO: rcu_tasks detected stalls on tasks: |
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And continues with the output of sched_show_task() for each |
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task stalling the current RCU-tasks grace period. |
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Interpreting RCU's CPU Stall-Detector "Splats" |
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============================================== |
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For non-RCU-tasks flavors of RCU, when a CPU detects that some other |
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CPU is stalling, it will print a message similar to the following:: |
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INFO: rcu_sched detected stalls on CPUs/tasks: |
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2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 |
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16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0 |
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(detected by 32, t=2603 jiffies, g=7075, q=625) |
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This message indicates that CPU 32 detected that CPUs 2 and 16 were both |
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causing stalls, and that the stall was affecting RCU-sched. This message |
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will normally be followed by stack dumps for each CPU. Please note that |
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PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that |
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the tasks will be indicated by PID, for example, "P3421". It is even |
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possible for an rcu_state stall to be caused by both CPUs *and* tasks, |
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in which case the offending CPUs and tasks will all be called out in the list. |
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In some cases, CPUs will detect themselves stalling, which will result |
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in a self-detected stall. |
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CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with |
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the RCU core for the past three grace periods. In contrast, CPU 16's "(0 |
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ticks this GP)" indicates that this CPU has not taken any scheduling-clock |
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interrupts during the current stalled grace period. |
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The "idle=" portion of the message prints the dyntick-idle state. |
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The hex number before the first "/" is the low-order 12 bits of the |
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dynticks counter, which will have an even-numbered value if the CPU |
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is in dyntick-idle mode and an odd-numbered value otherwise. The hex |
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number between the two "/"s is the value of the nesting, which will be |
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a small non-negative number if in the idle loop (as shown above) and a |
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very large positive number otherwise. |
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The "softirq=" portion of the message tracks the number of RCU softirq |
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handlers that the stalled CPU has executed. The number before the "/" |
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is the number that had executed since boot at the time that this CPU |
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last noted the beginning of a grace period, which might be the current |
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(stalled) grace period, or it might be some earlier grace period (for |
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example, if the CPU might have been in dyntick-idle mode for an extended |
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time period). The number after the "/" is the number that have executed |
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since boot until the current time. If this latter number stays constant |
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across repeated stall-warning messages, it is possible that RCU's softirq |
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handlers are no longer able to execute on this CPU. This can happen if |
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the stalled CPU is spinning with interrupts are disabled, or, in -rt |
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kernels, if a high-priority process is starving RCU's softirq handler. |
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The "fqs=" shows the number of force-quiescent-state idle/offline |
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detection passes that the grace-period kthread has made across this |
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CPU since the last time that this CPU noted the beginning of a grace |
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period. |
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The "detected by" line indicates which CPU detected the stall (in this |
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case, CPU 32), how many jiffies have elapsed since the start of the grace |
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period (in this case 2603), the grace-period sequence number (7075), and |
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an estimate of the total number of RCU callbacks queued across all CPUs |
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(625 in this case). |
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In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed |
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for each CPU:: |
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0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1 |
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The "last_accelerate:" prints the low-order 16 bits (in hex) of the |
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jiffies counter when this CPU last invoked rcu_try_advance_all_cbs() |
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from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from |
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rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle |
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processing is enabled. |
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If the grace period ends just as the stall warning starts printing, |
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there will be a spurious stall-warning message, which will include |
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the following:: |
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INFO: Stall ended before state dump start |
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This is rare, but does happen from time to time in real life. It is also |
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possible for a zero-jiffy stall to be flagged in this case, depending |
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on how the stall warning and the grace-period initialization happen to |
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interact. Please note that it is not possible to entirely eliminate this |
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sort of false positive without resorting to things like stop_machine(), |
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which is overkill for this sort of problem. |
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If all CPUs and tasks have passed through quiescent states, but the |
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grace period has nevertheless failed to end, the stall-warning splat |
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will include something like the following:: |
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All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 |
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The "23807" indicates that it has been more than 23 thousand jiffies |
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since the grace-period kthread ran. The "jiffies_till_next_fqs" |
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indicates how frequently that kthread should run, giving the number |
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of jiffies between force-quiescent-state scans, in this case three, |
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which is way less than 23807. Finally, the root rcu_node structure's |
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->qsmask field is printed, which will normally be zero. |
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If the relevant grace-period kthread has been unable to run prior to |
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the stall warning, as was the case in the "All QSes seen" line above, |
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the following additional line is printed:: |
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rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 |
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Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. |
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Starving the grace-period kthreads of CPU time can of course result |
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in RCU CPU stall warnings even when all CPUs and tasks have passed |
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through the required quiescent states. The "g" number shows the current |
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grace-period sequence number, the "f" precedes the ->gp_flags command |
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to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the |
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kthread is waiting for a short timeout, the "state" precedes value of the |
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task_struct ->state field, and the "cpu" indicates that the grace-period |
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kthread last ran on CPU 5. |
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If the relevant grace-period kthread does not wake from FQS wait in a |
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reasonable time, then the following additional line is printed:: |
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kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 |
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The "23804" indicates that kthread's timer expired more than 23 thousand |
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jiffies ago. The rest of the line has meaning similar to the kthread |
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starvation case. |
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Additionally, the following line is printed:: |
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Possible timer handling issue on cpu=4 timer-softirq=11142 |
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Here "cpu" indicates that the grace-period kthread last ran on CPU 4, |
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where it queued the fqs timer. The number following the "timer-softirq" |
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is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not |
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change on successive RCU CPU stall warnings, there is further reason to |
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suspect a timer problem. |
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These messages are usually followed by stack dumps of the CPUs and tasks |
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involved in the stall. These stack traces can help you locate the cause |
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of the stall, keeping in mind that the CPU detecting the stall will have |
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an interrupt frame that is mainly devoted to detecting the stall. |
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Multiple Warnings From One Stall |
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================================ |
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If a stall lasts long enough, multiple stall-warning messages will |
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be printed for it. The second and subsequent messages are printed at |
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longer intervals, so that the time between (say) the first and second |
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message will be about three times the interval between the beginning |
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of the stall and the first message. It can be helpful to compare the |
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stack dumps for the different messages for the same stalled grace period. |
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Stall Warnings for Expedited Grace Periods |
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========================================== |
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If an expedited grace period detects a stall, it will place a message |
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like the following in dmesg:: |
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INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. |
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This indicates that CPU 7 has failed to respond to a reschedule IPI. |
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The three periods (".") following the CPU number indicate that the CPU |
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is online (otherwise the first period would instead have been "O"), |
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that the CPU was online at the beginning of the expedited grace period |
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(otherwise the second period would have instead been "o"), and that |
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the CPU has been online at least once since boot (otherwise, the third |
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period would instead have been "N"). The number before the "jiffies" |
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indicates that the expedited grace period has been going on for 21,119 |
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jiffies. The number following the "s:" indicates that the expedited |
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grace-period sequence counter is 73. The fact that this last value is |
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odd indicates that an expedited grace period is in flight. The number |
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following "root:" is a bitmask that indicates which children of the root |
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rcu_node structure correspond to CPUs and/or tasks that are blocking the |
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current expedited grace period. If the tree had more than one level, |
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additional hex numbers would be printed for the states of the other |
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rcu_node structures in the tree. |
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As with normal grace periods, PREEMPT_RCU builds can be stalled by |
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tasks as well as by CPUs, and that the tasks will be indicated by PID, |
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for example, "P3421". |
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It is entirely possible to see stall warnings from normal and from |
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expedited grace periods at about the same time during the same run.
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