linux-kernel - Re: [PATCH 6/6] sched/numa: Delay retrying placement for automatic NUMA balance after wake

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Date:   Tue, 13 Feb 2018 14:18:12 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] sched/numa: Delay retrying placement for automatic
 NUMA balance after wake_affine

On Tue, Feb 13, 2018 at 03:01:37PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 13, 2018 at 01:37:30PM +0000, Mel Gorman wrote:
> > +static void
> > +update_wa_numa_placement(struct task_struct *p, int prev_cpu, int target)
> > +{
> > +	unsigned long interval;
> > +
> > +	if (!static_branch_likely(&sched_numa_balancing))
> > +		return;
> > +
> > +	/* If balancing has no preference then continue gathering data */
> > +	if (p->numa_preferred_nid == -1)
> > +		return;
> > +
> > +	/*
> > +	 * If the wakeup is not affecting locality then it is neutral from
> > +	 * the perspective of NUMA balacing so continue gathering data.
> > +	 */
> > +	if (cpus_share_cache(prev_cpu, target))
> > +		return;
> 
> Dang, I wanted to mention this before, but it slipped my mind. The
> comment and code don't match.
> 
> Did you want to write:
> 
> 	if (cpu_to_node(prev_cpu) == cpu_to_node(target))
> 		return;
> 

Well, it was deliberate. While it's possible to be on the same memory
node and not sharing cache, the scheduler typically is more concerned with
the LLC than NUMA per-se. If they share LLC, then I also assume that they
share memory locality.

-- 
Mel Gorman
SUSE Labs