linux-kernel - Re: [PATCH v2] sched/cpuset: distribute tasks within affinity masks

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Date:   Tue, 17 Mar 2020 14:35:32 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        cgroups@...r.kernel.org, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2] sched/cpuset: distribute tasks within affinity masks

On Wed, Mar 11, 2020 at 7:05 AM Qais Yousef <qais.yousef@....com> wrote:
>
> This actually helps me fix a similar problem I faced in RT [1]. If multiple RT
> tasks wakeup at the same time we get a 'thundering herd' issue where they all
> end up going to the same CPU, just to be pushed out again.
>
> Beside this will help fix another problem for RT tasks fitness, which is
> a manifestation of the problem above. If two tasks wake up at the same time and
> they happen to run on a little cpu (but request to run on a big one), one of
> them will end up being migrated because find_lowest_rq() will return the first
> cpu in the mask for both tasks.
>
> I tested the API (not the change in sched/core.c) and it looks good to me.

Nice, glad that the API already has another use case. Thanks for taking a look.

> nit: cpumask_first_and() is better here?

Yea, I would also prefer to use it, but the definition of
cpumask_first_and() follows this section, as it itself uses
cpumask_next_and().

> It might be a good idea to split the API from the user too.

Not sure what you mean by this, could you clarify?

On Tue, Mar 17, 2020 at 12:24 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > Anyway, for the API.
> >
> > Reviewed-by: Qais Yousef <qais.yousef@....com>
> > Tested-by: Qais Yousef <qais.yousef@....com>
>
> Thanks guys!

Thanks Peter, any other comments or are you happy with merging this patch as-is?