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

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Message-ID: <20180213150020.xds3htkutdk4sdwx@techsingularity.net>
Date:   Tue, 13 Feb 2018 15:00:20 +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 03:43:26PM +0100, Peter Zijlstra wrote:
> > 
> > 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.
> 
> True, but the remaining code only has effect for numa balance, which is
> concerned with nodes. So I don't see the point of using something
> potentially smaller.
> 
> Suppose someone did hardware where a node has 2 cache clusters, then
> we'd still set a wake_affine back-off for numa-balance, even though it
> remains on the same node.
> 
> How would that be useful?

Fair point, it could be unexpected from a NUMA balancing perspective and
sub-numa clustering does exist so it's a potential issue. I'm happy to
change it to cpu_to_node. I can resend the series if you prefer but feel
free to change it in-place if you're picking it up. I do not expect any
change on the machines I tested with as for all of them LLC was equivalent
to checking the node ID.

-- 
Mel Gorman
SUSE Labs