linux-kernel - Re: [PATCH v2] sched/fair: fix 1 task per CPU

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Date:   Thu, 13 Dec 2018 16:14:46 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2] sched/fair: fix 1 task per CPU

On Thu, 13 Dec 2018 at 14:12, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Dec 13, 2018 at 12:04:20PM +0100, Vincent Guittot wrote:
> > On Thu, 13 Dec 2018 at 11:44, Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Mon, Sep 10, 2018 at 04:43:09PM +0200, Vincent Guittot wrote:
> > > > When CPUs have different capacity because of RT/DL tasks or
> > > > micro-architecture or max frequency differences, there are situation where
> > > > the imbalance is not correctly set to migrate waiting task on the idle CPU.
> > > >
> > > > The UC uses the force_balance case:
> > > >
> > > >       if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
> > > >           busiest->group_no_capacity)
> > > >               goto force_balance;
> > > >
> > > > But calculate_imbalance fails to set the right amount of load to migrate
> > > > a task because of the special condition:
> > > >
> > > >   busiest->avg_load <= sds->avg_load || local->avg_load >= sds->avg_load)
> > > >
> > > > Add in fix_small_imbalance, this special case that triggered the force
> > > > balance in order to make sure that the amount of load to migrate will be
> > > > enough.
> > >
> > > So I think this patch is going in the wrong direction for a number of
> > > reasons:
> > >
> > >  - we'd like to get rid of fix_small_imbalance(), and this adds to it;
> > >
> > >  - the whole load_per_task stuff is terminally broken, it _cannot_ work
> > >    right.
> > >
> > >
> > > What I've suggested in the past is parameterizing the load balancer and
> > > picking different criteria to balance on:
> >
> > This patch is clearly a fix of the current implementation.
> > What you suggest below makes sense but implies a significant rework in
> > the calculate_imbalance and the load_balancer in general and will need
> > more time to reach a stable state.
> > Nevertheless, I will have a look at that
> >
> > I imagine that your feedback for https://lkml.org/lkml/2018/10/2/283
> > will be the same ?
>
> No; those actually look ok. It is mostly that I really don't think
> load_per_task makes any kind of sense.
>
> It sorta works when all tasks are of the same weight, but if you start
> using nice -- or way worse, cgroups -- then the number is complete
> bollocks.

Yes. As soon as we goes out of a simple load balance UC, load_per_task
becomes a kind of default random choice