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authorSteven Rostedt (Red Hat) <rostedt@goodmis.org>2014-12-10 17:31:07 -0500
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-01-16 06:59:32 -0800
commit37b2a5a7c849b7242322688ce380d0a1cc1b1d0f (patch)
parentadfd114937f829e354a8d6d3bd13d02bec769715 (diff)
tracing/sched: Check preempt_count() for current when reading task->state
commit aee4e5f3d3abb7a2239dd02f6d8fb173413fd02f upstream. When recording the state of a task for the sched_switch tracepoint a check of task_preempt_count() is performed to see if PREEMPT_ACTIVE is set. This is because, technically, a task being preempted is really in the TASK_RUNNING state, and that is what should be recorded when tracing a sched_switch, even if the task put itself into another state (it hasn't scheduled out in that state yet). But with the change to use per_cpu preempt counts, the task_thread_info(p)->preempt_count is no longer used, and instead task_preempt_count(p) is used. The problem is that this does not use the current preempt count but a stale one from a previous sched_switch. The task_preempt_count(p) uses saved_preempt_count and not preempt_count(). But for tracing sched_switch, if p is current, we really want preempt_count(). I hit this bug when I was tracing sleep and the call from do_nanosleep() scheduled out in the "RUNNING" state. sleep-4290 [000] 537272.259992: sched_switch: sleep:4290 [120] R ==> swapper/0:0 [120] sleep-4290 [000] 537272.260015: kernel_stack: <stack trace> => __schedule (ffffffff8150864a) => schedule (ffffffff815089f8) => do_nanosleep (ffffffff8150b76c) => hrtimer_nanosleep (ffffffff8108d66b) => SyS_nanosleep (ffffffff8108d750) => return_to_handler (ffffffff8150e8e5) => tracesys_phase2 (ffffffff8150c844) After a bit of hair pulling, I found that the state was really TASK_INTERRUPTIBLE, but the saved_preempt_count had an old PREEMPT_ACTIVE set and caused the sched_switch tracepoint to show it as RUNNING. Link: http://lkml.kernel.org/r/20141210174428.3cb7542a@gandalf.local.home Acked-by: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Fixes: 01028747559a "sched: Create more preempt_count accessors" Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 files changed, 1 insertions, 1 deletions
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 67e1bbf83695..dc7bb01f580f 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -100,7 +100,7 @@ static inline long __trace_sched_switch_state(struct task_struct *p)
* For all intents and purposes a preempted task is a running task.
- if (task_preempt_count(p) & PREEMPT_ACTIVE)
+ if (preempt_count() & PREEMPT_ACTIVE)