linux-kernel - Re: [PATCH 10/19] sched/numa: Stop multiple tasks from moving to the cpu at the same time

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Message-ID: <20180605095138.cslv7wxp7iljqcoe@techsingularity.net>
Date:   Tue, 5 Jun 2018 10:51:38 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/19] sched/numa: Stop multiple tasks from moving to the
 cpu at the same time

On Mon, Jun 04, 2018 at 03:30:19PM +0530, Srikar Dronamraju wrote:
> Task migration under numa balancing can happen in parallel. More than
> one task might choose to migrate to the same cpu at the same time. This
> can result in
> - During task swap, choosing a task that was not part of the evaluation.
> - During task swap, task which just got moved into its preferred node,
>   moving to a completely different node.
> - During task swap, task failing to move to the preferred node, will have
>   to wait an extra interval for the next migrate opportunity.
> - During task movement, multiple task movements can cause load imbalance.
> 
> This problem is more likely if there are more cores per node or more
> nodes in the system.
> 
> Use a per run-queue variable to check if numa-balance is active on the
> run-queue.
> 

FWIW, I had noticed a similar problem when selecting an idle CPU for
SIS. When prototying something to look at nearby CPUs for cross-node
migrations during wake_affine I found that the patch allowed multiple
tasks to select the same CPU when a waker was waking many wakees and
ultimately dropped the patch. 

Either way for this patch;

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs