linux-kernel - Re: [PATCH v3 3/7] sched: rotate the cpu search window for better spread

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Date:   Fri, 28 Jun 2019 11:54:41 +0000
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, tglx@...utronix.de, steven.sistare@...cle.com,
        dhaval.giani@...cle.com, daniel.lezcano@...aro.org,
        vincent.guittot@...aro.org, viresh.kumar@...aro.org,
        tim.c.chen@...ux.intel.com, mgorman@...hsingularity.net
Subject: Re: [PATCH v3 3/7] sched: rotate the cpu search window for better
 spread

* subhra mazumdar <subhra.mazumdar@...cle.com> [2019-06-26 18:29:15]:

> Rotate the cpu search window for better spread of threads. This will ensure
> an idle cpu will quickly be found if one exists.

While rotating the cpu search window is good, not sure if this can find a
idle cpu quickly. The probability of finding an idle cpu still should remain
the same. No?

> 
> Signed-off-by: subhra mazumdar <subhra.mazumdar@...cle.com>
> ---
>  kernel/sched/fair.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> @@ -6219,9 +6219,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  		}
>  	}
>  
> +	if (per_cpu(next_cpu, target) != -1)
> +		target_tmp = per_cpu(next_cpu, target);
> +	else
> +		target_tmp = target;
> +
>  	time = local_clock();
>  
> -	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
> +	for_each_cpu_wrap(cpu, sched_domain_span(sd), target_tmp) {
> +		per_cpu(next_cpu, target) = cpu;

Shouldn't this assignment be outside the for loop.
With the current code,
1. We keep reassigning multiple times.
2. The last assignment happes for idle_cpu and sometimes the
assignment is for non-idle cpu.

>  		if (!--nr)
>  			return -1;
>  		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> -- 
> 2.9.3
> 

-- 
Thanks and Regards
Srikar Dronamraju