linux-kernel - Re: [patch] sched: fix broken smt/mc optimizations with CFS

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Date:	Tue, 4 Sep 2007 16:46:26 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Chuck Ebbert <cebbert@...hat.com>, mingo@...e.hu
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>, mingo@...e.hu,
	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [patch] sched: fix broken smt/mc optimizations with CFS

On Tue, Sep 04, 2007 at 07:35:21PM -0400, Chuck Ebbert wrote:
> On 08/28/2007 06:27 PM, Siddha, Suresh B wrote:
> > Try to fix MC/HT scheduler optimization breakage again, with out breaking
> > the FUZZ logic.
> > 
> > First fix the check
> > 	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task)
> > with this
> > 	if (*imbalance < busiest_load_per_task)
> > 
> > As the current check is always false for nice 0 tasks (as SCHED_LOAD_SCALE_FUZZ
> > is same as busiest_load_per_task for nice 0 tasks).
> > 
> > With the above change, imbalance was getting reset to 0 in the corner case
> > condition, making the FUZZ logic fail. Fix it by not corrupting the
> > imbalance and change the imbalance, only when it finds that the
> > HT/MC optimization is needed.
> > 
> > Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
> > ---
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 9fe473a..03e5e8d 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -2511,7 +2511,7 @@ group_next:
> >  	 * a think about bumping its value to force at least one task to be
> >  	 * moved
> >  	 */
> > -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
> > +	if (*imbalance < busiest_load_per_task) {
> >  		unsigned long tmp, pwr_now, pwr_move;
> >  		unsigned int imbn;
> >  
> > @@ -2563,10 +2563,8 @@ small_imbalance:
> >  		pwr_move /= SCHED_LOAD_SCALE;
> >  
> >  		/* Move if we gain throughput */
> > -		if (pwr_move <= pwr_now)
> > -			goto out_balanced;
> > -
> > -		*imbalance = busiest_load_per_task;
> > +		if (pwr_move > pwr_now)
> > +			*imbalance = busiest_load_per_task;
> >  	}
> >  
> >  	return busiest;
> 
> Seems this didn't get merged? Latest git as of today still has the code
> as it was before this patch.

This is must fix for .23 and Ingo previously mentioned that he will push it
for .23

Ingo?
-
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