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Date:   Sun, 10 Oct 2021 00:50:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] sched/fair: Check idle_cpu in select_idle_core/cpu()

On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote:
> In select_idle_core(), the idle core returned may have no cpu
> allowed. I think the idle core returned for the task is the one
> that can be allowed to run. I insist on this semantics.
> 
> In select_idle_cpu(), if select_idle_core() can not find the
> idle core, one reason is that the core is not allowed for the
> task, but the core itself is idle from the point of
> sds->has_idle_cores. I insist on this semantics.
> 
> No others, just two additional check.
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f6a05d9b5443..a44aca5095d3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
>  			*idle_cpu = cpu;
>  	}
>  
> -	if (idle)
> +	if (idle && *idle_cpu != -1)
>  		return core;

In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks:

	(unsigned)i < nr_cpumask_bits

>  	cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
> @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  		}
>  	}
>  
> -	if (has_idle_core)
> +	if (has_idle_core && *idle_cpu != -1)
>  		set_idle_cores(target, false);

And this one I'm completely failing, why shouldn't we mark the core as
non-idle when there is a single idle CPU found? That's just worng.

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