linux-kernel - Re: [PATCH] sched/fair: Make sched-idle cpu selection consistent throughout

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Message-Id: <7d3a1549-a99c-ae42-6074-8ed2ecd7074f@linux.ibm.com>
Date:   Fri, 25 Oct 2019 12:13:27 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Make sched-idle cpu selection consistent
 throughout

Hi Viresh,

On 10/24/19 12:15 PM, Viresh Kumar wrote:
> There are instances where we keep searching for an idle CPU despite
> having a sched-idle cpu already (in find_idlest_group_cpu(),
> select_idle_smt() and select_idle_cpu() and then there are places where
> we don't necessarily do that and return a sched-idle cpu as soon as we
> find one (in select_idle_sibling()). This looks a bit inconsistent and
> it may be worth having the same policy everywhere.
> 
> On the other hand, choosing a sched-idle cpu over a idle one shall be
> beneficial from performance point of view as well, as we don't need to
> get the cpu online from a deep idle state which is quite a time
> consuming process and delays the scheduling of the newly wakeup task.
> 
> This patch tries to simplify code around sched-idle cpu selection and
> make it consistent throughout.
> 
> FWIW, tests were done with the help of rt-app (8 SCHED_OTHER and 5
> SCHED_IDLE tasks, not bound to any cpu) on ARM platform (octa-core), and
> no significant difference in scheduling latency of SCHED_OTHER tasks was
> found.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---

[...]

> @@ -5755,13 +5749,11 @@ static int select_idle_smt(struct task_struct *p, int target)
>  	for_each_cpu(cpu, cpu_smt_mask(target)) {
>  		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>  			continue;
> -		if (available_idle_cpu(cpu))
> +		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
>  			return cpu;

I guess this is a correct approach, but just wondering what if we still
keep searching for a sched_idle CPU even though we have found an
available_idle CPU?

[...]


Thanks,
Parth