Subject: Re: [BUG] How to get real-time priority using idle priority
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Mike Galbraith <efault@gmx.de>
Cc: Brian Rogers <brian@xyzw.org>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org
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Date: Thu, 15 Jan 2009 12:41:26 +0100
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On Thu, 2009-01-15 at 12:37 +0100, Mike Galbraith wrote:
> On Thu, 2009-01-15 at 11:30 +0100, Mike Galbraith wrote:
> > On Thu, 2009-01-15 at 11:14 +0100, Peter Zijlstra wrote:
> > >
> > > Which leads me to suggest the following
> > > 
> > > ---
> > > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > > index 8e1352c..f2d2d94 100644
> > > --- a/kernel/sched_fair.c
> > > +++ b/kernel/sched_fair.c
> > > @@ -283,7 +283,7 @@ static void update_min_vruntime(struct cfs_rq
> > > *cfs_rq)
> > >  						   struct sched_entity,
> > >  						   run_node);
> > >  
> > > -		if (vruntime == cfs_rq->min_vruntime)
> > > +		if (!cfs_rq->curr)
> > >  			vruntime = se->vruntime;
> > >  		else
> > >  			vruntime = min_vruntime(vruntime, se->vruntime);
> > 
> > Aha.  Yeah, I'll re-test with that instead.
> 
> Works a treat.

*cheer* lets get this merged asap, and CC -stable as well.

> > > The below can be split into 3 patches:
> > > 
> > >  - the idle weight change (do we really need that? why?)
> > 
> > I saw idle tasks slamming extremely far.  I'll verify, less is more.
> 
> time advanced in 100ms
> weight=2
> 64765.988352
> 67012.881408
> 88501.412352
> 
> weight=3
> 35496.181411
> 34130.971298
> 35497.411573
> 
> Measured from an RT shell doing..
>    while sleep .1; do cat /proc/sched_debug >> /debug; done
> ...for a pinned chew.  Not necessarily gnats-arse accurate, but good
> enough to see the margin of error is pretty high with weight=2.
> 
> Your call.

Right, 3 does look more stable, ok lets go with that.

Thanks Mike!

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