linux-kernel - Re: [PATCH 4/4] sched/fair: Do not migrate due to a sync wakeup on exit

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Date:   Mon, 12 Feb 2018 17:47:29 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] sched/fair: Do not migrate due to a sync wakeup on
 exit

On Mon, Feb 12, 2018 at 06:31:47PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 12, 2018 at 02:58:57PM +0000, Mel Gorman wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 28c8d9c91955..50442697b455 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5710,8 +5710,14 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
> >  	if (idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu))
> >  		return idle_cpu(prev_cpu) ? prev_cpu : this_cpu;
> >  
> > -	if (sync && cpu_rq(this_cpu)->nr_running == 1)
> > +	if (sync && cpu_rq(this_cpu)->nr_running == 1) {
> > +		/* Avoid tasks exiting pulling parents to new nodes */
> > +		if ((current->flags & PF_EXITING) &&
> > +		    !cpus_share_cache(this_cpu, prev_cpu))
> > +			return prev_cpu;
> > +
> 
> Cute, but should we not kill @sync right at the source in this case?
> 
> Something a little like this?
> 

That works too but note that it has a slightly disadvantage in that we
add weight to the select_task_rq_fair fast path for all wakeups, not just
those that are affine. It's also not 100% functionally equivalent as I was
still allowing parents to migrate to the CPU if it shared cache although I
expect any advantage there is marginal. I can replace my patch with yours
if you really prefer it as the overhead in the fast path should be marginal.

-- 
Mel Gorman
SUSE Labs