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Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware
To:     Steven Rostedt <rostedt@goodmis.org>,
        Qais Yousef <qais.yousef@arm.com>
Cc:     Pavan Kondeti <pkondeti@codeaurora.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <8868c5cf-e3f4-04e1-e071-0476ac813191@arm.com>
Date:   Mon, 3 Feb 2020 17:15:01 +0000
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On 03/02/2020 16:14, Steven Rostedt wrote:
> On Mon, 3 Feb 2020 14:27:14 +0000
> Qais Yousef <qais.yousef@arm.com> wrote:
> 
>> I don't see one right answer here. The current mechanism could certainly do
>> better; but it's not clear what better means without delving into system
>> specific details. I am open to any suggestions to improve it.
> 
> The way I see this is that if there's no big cores available but little
> cores are, and the RT task has those cores in its affinity mask then
> the task most definitely should consider moving to the little core. The
> cpu_find() should return them!
> 
> But, what we can do is to mark the little core that's running an RT
> task on a it that prefers bigger cores, as "rt-overloaded".  This will
> add this core into the being looked at when another core schedules out
> an RT task. When that happens, the RT task on the little core will get
> pulled back to the big core.
> 

That sounds sensible enough - it's also very similar to what we have for
CFS, labeled under "misfit tasks" (i.e. tasks that are "too big" for
LITTLEs).

> 
> Note, this will require a bit more logic as the overloaded code wasn't
> designed for migration of running tasks, but that could be added.
> 

I haven't adventured too much within RT land, but FWIW that's what we use
the CPU stopper for in CFS (see active_load_balance_cpu_stop()).

> -- Steve
>