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From:   Wei Wang <wvw@google.com>
Date:   Tue, 22 Sep 2020 10:14:46 -0700
Message-ID: <CAGXk5yp4ubupTKDg1s+ZbK3cj_aKh9pQJzshWoZ3CvqnQLJX=A@mail.gmail.com>
Subject: Re: [PATCH] sched/rt: Disable RT_RUNTIME_SHARE by default
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "J . Avila" <elavila@google.com>, Todd Kjos <tkjos@google.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>
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On Mon, Sep 21, 2020 at 7:40 AM Daniel Bristot de Oliveira
<bristot@redhat.com> wrote:
>
> The RT_RUNTIME_SHARE sched feature enables the sharing of rt_runtime
> between CPUs, allowing a CPU to run a real-time task up to 100% of the
> time while leaving more space for non-real-time tasks to run on the CPU
> that lend rt_runtime.
>
> The problem is that a CPU can easily borrow enough rt_runtime to allow
> a spinning rt-task to run forever, starving per-cpu tasks like kworkers,
> which are non-real-time by design.
>
> This patch disables RT_RUNTIME_SHARE by default, avoiding this problem.
> The feature will still be present for users that want to enable it,
> though.
>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: linux-kernel@vger.kernel.org
> ---
>  kernel/sched/features.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 7481cd96f391..68d369cba9e4 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -77,7 +77,7 @@ SCHED_FEAT(WARN_DOUBLE_CLOCK, false)
>  SCHED_FEAT(RT_PUSH_IPI, true)
>  #endif
>
> -SCHED_FEAT(RT_RUNTIME_SHARE, true)
> +SCHED_FEAT(RT_RUNTIME_SHARE, false)
>  SCHED_FEAT(LB_MIN, false)
>  SCHED_FEAT(ATTACH_AGE_LOAD, true)
>
> --
> 2.26.2
>

Tested on an Android device and can no longer see long running RT
tasks (yes, Android have those for reasons).