* Steven Rostedt <rost...@goodmis.org> wrote:
>
> 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.
>
> Cc: sta...@vger.kernel.org # 3.13+
> Fixes: 01028747559a "sched: Create more preempt_count accessors"
> Signed-off-by: Steven Rostedt <rost...@goodmis.org>
> ---
> include/trace/events/sched.h | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 0a68d5ae584e..13fbadcc172b 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -97,10 +97,14 @@ static inline long __trace_sched_switch_state(struct
> task_struct *p)
> long state = p->state;
>
> #ifdef CONFIG_PREEMPT
> + unsigned long pc;
> +
> + pc = (p == current) ? preempt_count() : task_preempt_count(p);
> +
> /*
> * For all intents and purposes a preempted task is a running task.
> */
> - if (task_preempt_count(p) & PREEMPT_ACTIVE)
> + if (pc & PREEMPT_ACTIVE)
> state = TASK_RUNNING | TASK_STATE_MAX;
I really don't like the overhead around here.
Thanks,
Ingo
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