[PATCH 2/2] tracing/sched: Check preempt_count() for current when reading task->state
From: Steven Rostedt
Date: Fri Dec 12 2014 - 23:22:50 EST
From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
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  537272.259992: sched_switch: sleep:4290  R ==> swapper/0:0 
sleep-4290  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.
Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 3.13+
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Fixes: 01028747559a "sched: Create more preempt_count accessors"
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
include/trace/events/sched.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 0a68d5ae584e..a7d67bc14906 100644
@@ -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)
state = TASK_RUNNING | TASK_STATE_MAX;
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