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diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt deleted file mode 100644 index f48f4621ccbc..000000000000 --- a/Documentation/RCU/stallwarn.txt +++ /dev/null @@ -1,321 +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. - -o A CPU looping with bottom halves disabled. - -o 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(). - -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. It can also - result in the "rcu_.*kthread starved for" console-log message, - which will include additional debugging information. - -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. - - 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. - -o 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. - -o 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! - -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, 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 Nonlazy posted: ..D - -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:" indicates lazy-callback -status, so that an "l" indicates that all callbacks were lazy at the start -of the last idle period and an "L" indicates that there are currently -no non-lazy callbacks (in both cases, "." 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 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. |