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documentation: Add verbiage on RCU-tasks stall warning messages 

This commit documents RCU-tasks stall warning messages and also describes
when to use the new cond_resched_rcu_qs() API.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2014-07-29 09:49:23 -07:00
parent c7b24d2b9a
commit 37fe5f0e27
1 changed files with 24 additions and 9 deletions
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33
Documentation/RCU/stallwarn.txt
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@ -56,8 +56,20 @@ RCU_STALL_RAT_DELAY
two jiffies. (This is a cpp macro, not a kernel configuration
parameter.)
When a CPU detects that it is stalling, it will print a message similar
to the following:
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 wtih 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.
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)
@ -174,8 +186,12 @@ o A CPU looping with preemption disabled. This condition can
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().
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 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
@ -208,11 +224,10 @@ o A hardware failure. This is quite unlikely, but has occurred
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-bh implementations have CPU stall warning.
SRCU does not have its own CPU stall warnings, but its calls to
synchronize_sched() will result in RCU-sched detecting RCU-sched-related
CPU stalls. Please note that RCU only detects CPU stalls when there is
a grace period in progress. No grace period, no CPU stall warnings.
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.