Subject: Re: [patch] Re: scheduler oddity [bug?]
From: Mike Galbraith <efault@gmx.de>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>, Balazs Scheidler <bazsi@balabit.hu>,
       linux-kernel@vger.kernel.org, Willy Tarreau <w@1wt.eu>
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Date: Mon, 09 Mar 2009 18:28:14 +0100
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On Mon, 2009-03-09 at 17:12 +0100, Peter Zijlstra wrote:
> On Mon, 2009-03-09 at 16:30 +0100, Mike Galbraith wrote:
> > +static void put_prev_task(struct rq *rq, struct task_struct *prev)
> > +{
> > +       if (prev->state == TASK_RUNNING) {
> > +               u64 runtime = prev->se.sum_exec_runtime;
> > +
> > +               runtime -= prev->se.prev_sum_exec_runtime;
> > +               runtime = min_t(u64, runtime, 2*sysctl_sched_migration_cost);
> > +
> > +               /*
> > +                * In order to avoid avg_overlap growing stale when we are
> > +                * indeed overlapping and hence not getting put to sleep, grow
> > +                * the avg_overlap on preemption.
> > +                */
> > +               update_avg(&prev->se.avg_overlap, runtime);
> > +       }
> > +       prev->sched_class->put_prev_task(rq, prev);
> > +}
> 
> Right, so we both found it worked quite well, I'm still slightly puzzled
> but it.
> 
> If something gets preempted a lot and will therefore have short runtimes
> it will be seen as sync even though it might not at all be.

Yes, and the netperf on 2 CPUs with shared cache numbers show that's
happening.  It just so happens that in the non-shared case, netperf's
cache pain far outweighs the benefit of having more CPU available :-/

	-Mike

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