linux-kernel - Re: [PATCH V3 0/2] sched/fair: Fallback to sched-idle CPU in absence of idle CPUs

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Message-ID: <20190701134343.GT3402@hirez.programming.kicks-ass.net>
Date:   Mon, 1 Jul 2019 15:43:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>, tkjos@...gle.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        quentin.perret@...aro.org, chris.redpath@....com,
        steven.sistare@...cle.com, subhra.mazumdar@...cle.com,
        songliubraving@...com
Subject: Re: [PATCH V3 0/2] sched/fair: Fallback to sched-idle CPU in absence
 of idle CPUs

On Wed, Jun 26, 2019 at 10:36:28AM +0530, Viresh Kumar wrote:
> Hi,
> 
> We try to find an idle CPU to run the next task, but in case we don't
> find an idle CPU it is better to pick a CPU which will run the task the
> soonest, for performance reason.
> 
> A CPU which isn't idle but has only SCHED_IDLE activity queued on it
> should be a good target based on this criteria as any normal fair task
> will most likely preempt the currently running SCHED_IDLE task
> immediately. In fact, choosing a SCHED_IDLE CPU over a fully idle one
> shall give better results as it should be able to run the task sooner
> than an idle CPU (which requires to be woken up from an idle state).
> 
> This patchset updates both fast and slow paths with this optimization.

So this basically does the trivial SCHED_IDLE<-* wakeup preemption test;
one could consider doing the full wakeup preemption test instead.

Now; the obvious argument against doing this is cost; esp. the cgroup
case is very expensive I suppose. But it might be a fun experiment to
try.

That said; I'm tempted to apply these patches..