linux-kernel - RE: [PATCH] sched: prevent compiler from optimising sched_avg

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Date:	Tue, 23 Mar 2010 19:10:18 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	'Eric Dumazet' <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org,
	Catalin Marinas <Catalin.Marinas@....com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: [PATCH] sched: prevent compiler from optimising
 sched_avg_update loop

On Tue, 2010-03-23 at 18:03 +0000, Will Deacon wrote:
> Hello Eric,
> 
> Thanks for looking at the patch.
> 
> > > diff --git a/kernel/sched.c b/kernel/sched.c
> > > index 9ab3cd7..6b74f21 100644
> > > --- a/kernel/sched.c
> > > +++ b/kernel/sched.c
> > > @@ -1238,11 +1238,10 @@ static u64 sched_avg_period(void)
> > >  static void sched_avg_update(struct rq *rq)
> > >  {
> > >  	s64 period = sched_avg_period();
> > > +	s64 elapsed_periods = div_s64(rq->clock - rq->age_stamp - 1, period);
> > >
> > > -	while ((s64)(rq->clock - rq->age_stamp) > period) {
> > > -		rq->age_stamp += period;
> > > -		rq->rt_avg /= 2;
> > > -	}
> > > +	rq->age_stamp += (u64)(elapsed_periods * period);
> > > +	rq->rt_avg >>= elapsed_periods;
> > >  }
> > >
> > >  static void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
> > 
> > Please take a look at __iter_div_u64_rem() , because we had a similar
> > problem in the past. We want to avoid this div_s64() call.
> 
> Yes, I saw the inline assembly fix there. I avoided that fix because
> I was trying not to execute the loop body multiple times. Is the iterative
> approach preferred over a single call to div_s64? I don't have a handle on
> how many iterations are typically executed for this loop.

I expect it to be mostly 0 and occasionally 1 cycle, except when someone
pokes at a sysctl with funny values, at which point it might go round
much much faster.
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