linux-kernel - Re: [PATCH 16/19] sched/numa: Detect if node actively handling migration

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Date:   Wed, 06 Jun 2018 09:55:14 -0400
From:   Rik van Riel <riel@...riel.com>
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>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 16/19] sched/numa: Detect if node actively handling
 migration

On Wed, 2018-06-06 at 05:55 -0700, Srikar Dronamraju wrote:
> > > 
> > > While we can't complete avoid this, the second check will try to
> > > make
> > > sure we don't hop on/hop off just for small incremental numa
> > > improvement.
> > 
> > However, all those racing tasks start searching
> > the CPUs on a node from the same start position.
> > 
> > That means they may all get stuck on the same
> > task/cpu A, and not select the better task/cpu B.
> > 
> > What am I missing? 
> 
> All tasks will not be stuck at task/cpu A.
> 
> "[PATCH 10/19] sched/numa: Stop multiple tasks from moving to the
> cpu..." the first task to set cpu A as swap target will ensure
> subsequent tasks wont be allowed to set cpu A as target for swap till
> it
> finds a better task/cpu. Because of this there a very very small
> chance
> of a second task unable to find a task to swap.

Would it not be better for task_numa_compare to skip
from consideration CPUs that somebody else is already
trying to migrate a task to, but still search for the
best location, instead of settling for a worse location
to migrate to?

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