linux-kernel - Re: scheduler oddity [bug?]

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Message-ID: <20090308175255.GA22802@elte.hu>
Date:	Sun, 8 Mar 2009 18:52:55 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Galbraith <efault@....de>
Cc:	Balazs Scheidler <bazsi@...abit.hu>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: scheduler oddity [bug?]


* Mike Galbraith <efault@....de> wrote:

> On Sun, 2009-03-08 at 16:39 +0100, Ingo Molnar wrote:
> > * Mike Galbraith <efault@....de> wrote:
> > 
> > > The problem with your particular testcase is that while one 
> > > half has an avg_overlap (what we use as affinity hint for 
> > > synchronous wakeups) which triggers the affinity hint, the 
> > > other half has avg_overlap of zero, what it was born with, so 
> > > despite significant execution overlap, the scheduler treats 
> > > them as if they were truly synchronous tasks.
> > 
> > hm, why does it stay on zero?
> 
> Wakeup preemption.  Presuming here: heavy task wakes light 
> task, is preempted, light task stuffs data into pipe, heavy 
> task doesn't block, so no avg_overlap is ever computed.  The 
> heavy task uses 100% CPU.
> 
> Running as SCHED_BATCH (virgin source), it becomes sane.

ah.

I'd argue then that time spent on the rq preempted _should_ 
count in avg_overlap statistics. I.e. couldnt we do something 
like ... your patch? :)

> >     if (sleep && p->se.last_wakeup) {
> >             update_avg(&p->se.avg_overlap,
> >                        p->se.sum_exec_runtime - p->se.last_wakeup);
> >             p->se.last_wakeup = 0;
> > -   }
> > +   } else if (p->se.avg_overlap < limit && runtime >= limit)
> > +           update_avg(&p->se.avg_overlap, runtime);

Just done unconditionally, i.e. something like:

	if (sleep) {
		runtime = p->se.sum_exec_runtime - p->se.last_wakeup;
		p->se.last_wakeup = 0;
	} else {
		runtime = p->se.sum_exec_runtime - p->se.prev_sum_exec_runtime;
	}

	update_avg(&p->se.avg_overlap, runtime);

?

	Ingo
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