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: <20190703091331.gnoouol3hn77r65b@vireshk-i7>
Date:   Wed, 3 Jul 2019 14:43:31 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Peter Zijlstra <peterz@...radead.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 01-07-19, 15:43, Peter Zijlstra wrote:
> 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;

Right.

> one could consider doing the full wakeup preemption test instead.

I am not sure what you meant by "full wakeup preemption test" :(

> 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..

Please do, who is stopping you :)

-- 
viresh