Re: [PATCH] tracing/sched: Check preempt_count() for current when reading task->state

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 11 Dec 2014 13:31:21 +0100
Ingo Molnar <mingo@kernel.org> 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