Subject: Re: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU

From: Mel Gorman <[email-masked]>
Date: Tue, 8 Dec 2020 10:59:00 +0000

On Tue, Dec 08, 2020 at 11:07:19AM +0100, Dietmar Eggemann wrote:
> On 07/12/2020 10:15, Mel Gorman wrote:
> > SIS_AVG_CPU was introduced as a means of avoiding a search when the
> > average search cost indicated that the search would likely fail. It
> > was a blunt instrument and disabled by 4c77b18cf8b7 ("sched/fair: Make
> > select_idle_cpu() more aggressive") and later replaced with a proportional
> > search depth by 1ad3aaf3fcd2 ("sched/core: Implement new approach to
> > scale select_idle_cpu()").
> > 
> > While there are corner cases where SIS_AVG_CPU is better, it has now been
> > disabled for almost three years. As the intent of SIS_PROP is to reduce
> > the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
> > on SIS_PROP as a throttling mechanism.
> > 
> > Signed-off-by: Mel Gorman <[email-masked]>
> > ---
> >  kernel/sched/fair.c     | 3 ---
> >  kernel/sched/features.h | 1 -
> >  2 files changed, 4 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 98075f9ea9a8..23934dbac635 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6161,9 +6161,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >  	avg_idle = this_rq()->avg_idle / 512;
> >  	avg_cost = this_sd->avg_scan_cost + 1;
> >  
> > -	if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
> > -		return -1;
> > -
> >  	if (sched_feat(SIS_PROP)) {
> >  		u64 span_avg = sd->span_weight * avg_idle;
> >  		if (span_avg > 4*avg_cost)
> 
> Nitpick:
> 
> Since now avg_cost and avg_idle are only used w/ SIS_PROP, they could go
> completely into the SIS_PROP if condition.
> 

Yeah, I can do that. In the initial prototype, that happened in a
separate patch that split out SIS_PROP into a helper function and I
never merged it back. It's a trivial change.

Thanks.

-- 
Mel Gorman
SUSE Labs

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