linux-kernel - Re: SD_LOAD

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Message-ID: <alpine.DEB.2.22.394.2010121317470.2901@hadrien>
Date:   Mon, 12 Oct 2020 13:18:00 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Vincent Guittot <vincent.guittot@...aro.org>
cc:     Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Gilles Muller <Gilles.Muller@...ia.fr>
Subject: Re: SD_LOAD_BALANCE



On Mon, 12 Oct 2020, Vincent Guittot wrote:

> On Mon, 12 Oct 2020 at 12:34, Julia Lawall <julia.lawall@...ia.fr> wrote:
> >
> > > > Would it be useful to always check whether prev is idle, perhaps in
> > > > wake_affine_idle or perhaps in select_idle_sibling?
> > >
> > > Yes, that would make sense to add a condition in  wake_affine_idle to
> > > return prev if this cpu is not idle (or about to become idle)
> >
> > The case where this cpu is idle would be in the interrupt case.  If both
> > prev cpu and this cpu are idle, is it more desirable to move the thread to
> > this cpu or to leave it where it was?
>
> I think that we should keep the current behavior regarding this cpu
> and the shared cache case and add one more test before the last return
> of the function.
>
> right now, we select this_cpu:
> -if this cpu is idle,  it shares cache with prev and previous is not idle.
> -if it's a sync wake up because we expect the task to use local data
> of the current running task on this cpu.
>
> Then we add a new case to return prev cpu if it is idle which is your case

OK, sounds good, thanks.

julia