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doc: Update stallwarn.rst with recent changes

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This commit calls out the possibility of self-detected stalls, adds new
messages, and calls out the use for stack traces.

Reported-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney
2021-07-14 11:46:55 -07:00
parent c28adacc14
commit 99c0974ffe
1 changed files with 15 additions and 6 deletions
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21
Documentation/RCU/stallwarn.rst
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@@ -189,8 +189,8 @@ rcupdate.rcu_task_stall_timeout
Interpreting RCU's CPU Stall-Detector "Splats" Interpreting RCU's CPU Stall-Detector "Splats"
============================================== ==============================================
For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling, For non-RCU-tasks flavors of RCU, when a CPU detects that some other
it will print a message similar to the following:: CPU is stalling, it will print a message similar to the following::
INFO: rcu_sched detected stalls on CPUs/tasks: INFO: rcu_sched detected stalls on CPUs/tasks:
2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
@@ -204,6 +204,8 @@ 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 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, 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. in which case the offending CPUs and tasks will all be called out in the list.
In some cases, CPUs will detect themselves stalling, which will result
in a self-detected stall.
CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with 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 the RCU core for the past three grace periods. In contrast, CPU 16's "(0
@@ -283,7 +285,8 @@ 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 stall warning, as was the case in the "All QSes seen" line above,
the following additional line is printed:: the following additional line is printed::
kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
Starving the grace-period kthreads of CPU time can of course result 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 in RCU CPU stall warnings even when all CPUs and tasks have passed
@@ -313,15 +316,21 @@ is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not
change on successive RCU CPU stall warnings, there is further reason to change on successive RCU CPU stall warnings, there is further reason to
suspect a timer problem. suspect a timer problem.
These messages are usually followed by stack dumps of the CPUs and tasks
involved in the stall. These stack traces can help you locate the cause
of the stall, keeping in mind that the CPU detecting the stall will have
an interrupt frame that is mainly devoted to detecting the stall.
Multiple Warnings From One Stall Multiple Warnings From One Stall
================================ ================================
If a stall lasts long enough, multiple stall-warning messages will be If a stall lasts long enough, multiple stall-warning messages will
printed for it. The second and subsequent messages are printed at be printed for it. The second and subsequent messages are printed at
longer intervals, so that the time between (say) the first and second longer intervals, so that the time between (say) the first and second
message will be about three times the interval between the beginning message will be about three times the interval between the beginning
of the stall and the first message. of the stall and the first message. It can be helpful to compare the
stack dumps for the different messages for the same stalled grace period.
Stall Warnings for Expedited Grace Periods Stall Warnings for Expedited Grace Periods