linux-kernel - Re: [RFC PATCH 9/9] sched: rotate the cpu search window for better spread

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Message-Id: <0931a2e0-0fbf-be64-d6c0-cba7745bb152@linux.ibm.com>
Date:   Thu, 5 Sep 2019 12:07:47 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     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, patrick.bellasi@....com
Subject: Re: [RFC PATCH 9/9] sched: rotate the cpu search window for better
 spread



On 8/30/19 11:19 PM, subhra mazumdar wrote:
> Rotate the cpu search window for better spread of threads. This will ensure
> an idle cpu will quickly be found if one exists.
> 
> Signed-off-by: subhra mazumdar <subhra.mazumdar@...cle.com>
> ---
>  kernel/sched/fair.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 94dd4a32..7419b47 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	u64 avg_cost, avg_idle;
>  	u64 time, cost;
>  	s64 delta;
> -	int cpu, floor, nr = INT_MAX;
> +	int cpu, floor, target_tmp, nr = INT_MAX;
>  
>  	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>  	if (!this_sd)
> @@ -6213,9 +6213,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  			nr = floor;
>  	}
>  
> +	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;

Is it possible that two simultaneous select_idle_cpu call have the same target value?

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