From 37fe5f0e2713608573c5df5e529e13a135625629 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 29 Jul 2014 09:49:23 -0700 Subject: [PATCH] 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 --- Documentation/RCU/stallwarn.txt | 33 ++++++++++++++++++++++++--------- 1 file changed, 24 insertions(+), 9 deletions(-) diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt index 68fe3ad27015..ef5a2fd4ff70 100644 --- a/Documentation/RCU/stallwarn.txt +++ b/Documentation/RCU/stallwarn.txt @@ -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.