linux-kernel - Re: [PATCH] tracing: Add task activate/deactivate tracepoints

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Message-ID: <20100531143622.GA5157@nowhere>
Date:	Mon, 31 May 2010 16:36:28 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] tracing: Add task activate/deactivate tracepoints

On Mon, May 31, 2010 at 10:54:59AM +0200, Peter Zijlstra wrote:
> On Mon, 2010-05-31 at 10:12 +0200, Peter Zijlstra wrote:
> > On Mon, 2010-05-31 at 10:00 +0200, Ingo Molnar wrote:
> > > > 
> > > > NAK, aside from a few corner cases wakeup and sleep are the important 
> > > > points.
> > > > 
> > > > The activate and deactivate functions are implementation details.
> > > 
> > > Frederic, can you show us a concrete example of where we dont know what is 
> > > going on due to inadequate instrumentation? Can we fix that be extending the 
> > > existing tracepoints? 
> > 
> > Right, so a few of those corner cases I mentioned above are things like
> > re-nice, PI-boosts etc.. Those use deactivate, modify task-state,
> > activate cycles. so if you want to see those, we can add an explicit
> > tracepoint for those actions.
> > 
> > An explicit nice/PI-boost tracepoint is much clearer than trying to
> > figure out wth the deactivate/activate cycle was for.
> 
> Another advantage of explicit tracepoints is that you'd see them even
> for non-running tasks, because we only do the deactivate/activate thingy
> for runnable tasks.

Yeah. So I agree with you that activate/deactivate are too much
implementation related, they even don't give much sense as we
don't know the cause of the event, could be a simple renice, or
could be a sleep.

So agreed, this sucks.

For the corner cases like re-nice and PI-boost or so, we can indeed plug
some higher level tracepoints there.

But there is one more important problem these tracepoints were solving and
that still need something:

We don't know when a task goes to sleep. We have two wait tracepoints,
sched_wait_task() to wait for a task to unschedule, and sched_process_wait()
that is a hooks for waitid and wait4 syscalls. So we are missing all
the event waiting from inside the kernel. But even with that, wait and sleep
doesn't mean the same thing. Sleeping don't always involve using the waiting
API.

  I think we need such tracepoint:

diff --git a/kernel/sched.c b/kernel/sched.c
index 8c0b90d..5f67c04 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3628,8 +3628,10 @@ need_resched_nonpreemptible:
 	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
 		if (unlikely(signal_pending_state(prev->state, prev)))
 			prev->state = TASK_RUNNING;
-		else
+		else {
+			trace_sched_task_sleep(prev);
 			deactivate_task(rq, prev, DEQUEUE_SLEEP);
+		}
 		switch_count = &prev->nvcsw;
 	}

  And if people need tracepoints in the events waiting API, we can add that
  later.

And concerning the task waking up, if it is not migrated, it means it stays
on its orig cpu. This is something that can be dealt from the post-processing.

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