On Thu, 11 Dec 2014 13:31:21 +0100
Ingo Molnar <[email protected]> wrote:
> > What overhead are you worried about? Note, this is in the
> > schedule tracepoint and does not affect the scheduler itself
> > (as long as the tracepoint is not enabled).
>
> Scheduler tracepoints are pretty popular, so I'm worried about
> their complexity when they are activated.
Understood.
>
> > I'm also thinking that as long as "prev" is always guaranteed
> > to be "current" we can remove the check and just use
> > preempt_count() always. But I'm worried that we can't
> > guaranteed that.
>
> You could add a WARN_ON_ONCE() or so to double check that
> assumption?
I actually thought about that, but that gives us the same overhead as
the branch we want to remove.
But if you are going for simpler, then that would make sense.
>
> > What other ideas do you have? Because wrong data is worse than
> > the overhead of the above code. If Thomas taught me anything,
> > it's that!
>
> My idea is to have simpler, yet correct code. And ponies!
So something like this instead?
-- Steve
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 0a68d5ae584e..782018b135ff 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -97,10 +97,12 @@ static inline long __trace_sched_switch_state(struct
task_struct *p)
long state = p->state;
#ifdef CONFIG_PREEMPT
+ WARN_ON_ONCE(p != current);
+
/*
* 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;
#endif
--
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