linux-kernel - Re: [PATCH v2 00/19] Fixes for sched/numa

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Date:   Mon, 23 Jul 2018 09:29:54 -0700
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 00/19]  Fixes for sched/numa_balancing

* Peter Zijlstra <peterz@...radead.org> [2018-07-23 17:21:47]:

> On Mon, Jul 23, 2018 at 08:09:55AM -0700, Srikar Dronamraju wrote:
> 
> > > >   sched/numa: Stop multiple tasks from moving to the cpu at the same time
> > 
> > This patch has go-ahead from Mel and Rik and no outstanding comments.
> 
> I left it out because it's part of the big xchg() mess.
> 
> In particular:
> 
> +       if (xchg(&rq->numa_migrate_on, 1))
> +               return;
> +
> +       if (env->best_cpu != -1) {
> +               rq = cpu_rq(env->best_cpu);
> +               WRITE_ONCE(rq->numa_migrate_on, 0);
> +       }
> 
> I'm again confused by clearing numa_migrate_on at this point..

First task choose a cpu to swap/migrate, sets the cpu to best_cpu and
also numa_migrate_on.  Next it finds a better cpu to swap/move. Now if
the task is able to move to the better cpu, then it should clear
numa_migrate_on on the previous best_cpu.

If we dont reset numa_migrate_on on finding a better cpu, the
numa_migrate_on stays set for the previous cpu, causing previous cpu to
never be a target of numa balance.


> 
> > > >   sched/numa: Updation of scan period need not be in lock
> > 
> > I didnt see any comments for this apart from an ack from Rik.
> > + It thought it was trivial and shouldnt have any side-effect.
> 
> Oh, my bad I actually have this one.
>