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Subject: Re: [PATCH] sched/rt: Disable RT_RUNTIME_SHARE by default
To:     Wei Wang <wvw@google.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>
References: <b776ab46817e3db5d8ef79175fa0d71073c051c7.1600697903.git.bristot@redhat.com>
 <CAGXk5yp4ubupTKDg1s+ZbK3cj_aKh9pQJzshWoZ3CvqnQLJX=A@mail.gmail.com>
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <64eca989-f2e7-033f-bb49-084f27c7209b@redhat.com>
Date:   Tue, 22 Sep 2020 21:04:18 +0200
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On 9/22/20 7:14 PM, Wei Wang wrote:
> 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).
> 

So:

Tested-by: Wei Wang <wvw@google.com>

?

Thanks!
-- Daniel