linux-kernel - Re: [PATCH 1/5] sched/fair: ignore SIS

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Date:   Thu, 14 Jul 2022 14:19:39 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     Abel Wu <wuyun.abel@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
CC:     <yangyicong@...ilicon.com>, Josh Don <joshdon@...gle.com>,
        Chen Yu <yu.c.chen@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core

On 2022/7/12 16:20, Abel Wu wrote:
> When SIS_UTIL is enabled, SIS domain scan will be skipped if
> the LLC is overloaded. Since the overloaded status is checked
> in the load balancing at LLC level, the interval is llc_size
> miliseconds. The duration might be long enough to affect the
> overall system throughput if idle cores are out of reach in
> SIS domain scan.
> 
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> ---
>  kernel/sched/fair.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a78d2e3b9d49..cd758b3616bd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6392,16 +6392,19 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  	struct sched_domain *this_sd;
>  	u64 time = 0;
>  
> -	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> -	if (!this_sd)
> -		return -1;
> -
>  	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>  
> -	if (sched_feat(SIS_PROP) && !has_idle_core) {
> +	if (has_idle_core)
> +		goto scan;
> +
> +	if (sched_feat(SIS_PROP)) {
>  		u64 avg_cost, avg_idle, span_avg;
>  		unsigned long now = jiffies;
>  
> +		this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> +		if (!this_sd)
> +			return -1;
> +

I don't follow the change here. True that this_sd is used only in SIS_PROP, but it seems irrelevant with your
commit. Does the position of this make any performance difference?

Thanks.

>  		/*
>  		 * If we're busy, the assumption that the last idle period
>  		 * predicts the future is flawed; age away the remaining
> @@ -6436,7 +6439,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  				return -1;
>  		}
>  	}
> -
> +scan:
>  	for_each_cpu_wrap(cpu, cpus, target + 1) {
>  		if (has_idle_core) {
>  			i = select_idle_core(p, cpu, cpus, &idle_cpu);
>