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diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt deleted file mode 100644 index 96a3d81837e1..000000000000 --- a/Documentation/RCU/stallwarn.txt +++ /dev/null @@ -1,276 +0,0 @@ -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: - -o A CPU looping in an RCU read-side critical section. - -o A CPU looping with interrupts disabled. - -o A CPU looping with preemption disabled. This condition can - result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh - stalls. - -o A CPU looping with bottom halves disabled. This condition can - result in RCU-sched and RCU-bh stalls. - -o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the - kernel without invoking schedule(). Note that cond_resched() - does not necessarily prevent RCU CPU stall warnings. Therefore, - if the looping in the kernel is really expected and desirable - behavior, you might need to replace some of the cond_resched() - calls with calls to cond_resched_rcu_qs(). - -o 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. - -o 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. - -o 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. - -o 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. - -o 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. - -o A bug in the RCU implementation. - -o 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, RCU-bh, 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 jiffies. 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_state detected stall on CPU 5 (t=2500 jiffies) - -This message indicates that CPU 5 detected that it was causing a stall, -and that the stall was affecting RCU-sched. This message will normally be -followed by a stack dump of the offending CPU. On TREE_RCU kernel builds, -RCU and RCU-sched are implemented by the same underlying mechanism, -while on PREEMPT_RCU kernel builds, RCU is instead implemented -by rcu_preempt_state. - -On the other hand, if the offending CPU fails to print out a stall-warning -message quickly enough, some other CPU will print a message similar to -the following: - -INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies) - -This message indicates that CPU 2 detected that CPUs 3 and 5 were both -causing stalls, and that the stall was affecting RCU-bh. 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 a rcu_preempt_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. - -Finally, if the grace period ends just as the stall warning starts -printing, there will be a spurious stall-warning message: - -INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies) - -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. - -Recent kernels will print a long form of the stall-warning message: - - INFO: rcu_preempt detected stall on CPU - 0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543 - (t=65000 jiffies) - -In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed: - - INFO: rcu_preempt detected stall on CPU - 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D - (t=65000 jiffies) - -The "(64628 ticks this GP)" indicates that this CPU has taken more -than 64,000 scheduling-clock interrupts during the current stalled -grace period. If the CPU was not yet aware of the current grace -period (for example, if it was offline), then this part of the message -indicates how many grace periods behind the CPU is. - -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 positive number if in the idle loop and a very large positive -number (as shown above) 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. - -For CONFIG_RCU_FAST_NO_HZ kernels, 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(). The "nonlazy_posted:" -prints the number of non-lazy callbacks posted since the last call to -rcu_needs_cpu(). Finally, an "L" indicates that there are currently -no non-lazy callbacks ("." is printed otherwise, as shown above) and -"D" indicates that dyntick-idle processing is enabled ("." is printed -otherwise, for example, if disabled via the "nohz=" kernel boot parameter). - -If the relevant grace-period kthread has been unable to run prior to -the stall warning, the following additional line is printed: - - rcu_preempt kthread starved for 2023 jiffies! - -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. - - -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: { 1 2 6 } 26009 jiffies s: 1043 - -This indicates that CPUs 1, 2, and 6 have failed to respond to a -reschedule IPI, that the expedited grace period has been going on for -26,009 jiffies, and that the expedited grace-period sequence counter is -1043. The fact that this last value is odd indicates that an expedited -grace period is in flight. - -It is entirely possible to see stall warnings from normal and from -expedited grace periods at about the same time from the same run. |