linux-kernel - Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found

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Message-ID: <20190510072125.GG2623@hirez.programming.kicks-ass.net>
Date:   Fri, 10 May 2019 09:21:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>, tkjos@...gle.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        quentin.perret@...aro.org, chris.redpath@....com,
        Dietmar.Eggemann@....com, linux-kernel@...r.kernel.org,
        steven.sistare@...cle.com
Subject: Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU
 isn't found

On Thu, Apr 25, 2019 at 03:07:40PM +0530, Viresh Kumar wrote:
> We target for an idle CPU in select_idle_sibling() to run the next task,
> but in case we don't find idle CPUs 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 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 patch updates the fast path to fallback to a sched-idle CPU if the
> idle CPU isn't found, the slow path can be updated separately later.
> 
> Following is the order in which select_idle_sibling() picks up next CPU
> to run the task now:
> 
> 1. idle_cpu(target) OR sched_idle_cpu(target)
> 2. idle_cpu(prev) OR sched_idle_cpu(prev)
> 3. idle_cpu(recent_used_cpu) OR sched_idle_cpu(recent_used_cpu)
> 4. idle core(sd)
> 5. idle_cpu(sd)
> 6. sched_idle_cpu(sd)
> 7. idle_cpu(p) - smt
> 8. sched_idle_cpu(p)- smt
> 
> Though the policy can be tweaked a bit if we want to have different
> priorities.

I don't hate his per se; but the whole select_idle_sibling() thing is
something that needs looking at.

There was the task stealing thing from Steve that looked interesting and
that would render your apporach unfeasible.