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

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Message-ID: <20180509163410.trju4eyrtslb3sk5@techsingularity.net>
Date:   Wed, 9 May 2018 17:34:10 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, hpa@...or.com, efault@....de,
        linux-kernel@...r.kernel.org, matt@...eblueprint.co.uk,
        peterz@...radead.org, ggherdovich@...e.cz,
        linux-tip-commits@...r.kernel.org, mpe@...erman.id.au
Subject: Re: [tip:sched/core] sched/numa: Delay retrying placement for
 automatic NUMA balance after wake_affine()

On Wed, May 09, 2018 at 03:58:14AM -0700, Srikar Dronamraju wrote:
> In the other scenario, where wake_affine set p->numa_migrate_preferred to a
> bigger value, the task calls numa_migrate_preferred(),
> numa_migrate_preferred  could be before p->numa_migrate_preferred. In
> such a case, we should have stopped the task from migration.
> However we overwrite p->numa_migrate_preferred and do the
> task_numa_migrate(). Somehow this doesn't seem to achieve what the
> commit intended.
> 
> Or did I misunderstand?
> 

Nope, the logic is broken. While it can be "fixed", the end result adds
complexity for very little gain. I believe the right path for the moment
is a revert and retry at a future date.

Thanks.

-- 
Mel Gorman
SUSE Labs