linux-kernel - Re: [PATCH 2/4] sched/fair: Move avg_scan_cost calculations under SIS

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Date:   Thu, 10 Dec 2020 13:18:05 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Peter Ziljstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Barry Song <song.bao.hua@...ilicon.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/4] sched/fair: Move avg_scan_cost calculations under
 SIS_PROP

On 2020/12/8 23:34, Mel Gorman wrote:
> As noted by Vincent Guittot, avg_scan_costs are calculated for SIS_PROP
> even if SIS_PROP is disabled. Move the time calculations under a SIS_PROP
> check and while we are at it, exclude the cost of initialising the CPU
> mask from the average scan cost.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  kernel/sched/fair.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ac7b34e7372b..5c41875aec23 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6153,6 +6153,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	if (!this_sd)
>  		return -1;
>  
> +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +
>  	if (sched_feat(SIS_PROP)) {
>  		u64 avg_cost, avg_idle, span_avg;
>  
> @@ -6168,11 +6170,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  			nr = div_u64(span_avg, avg_cost);
>  		else
>  			nr = 4;
> -	}
> -
> -	time = cpu_clock(this);
>  
> -	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +		time = cpu_clock(this);
> +	}
>  
>  	for_each_cpu_wrap(cpu, cpus, target) {
>  		if (!--nr)
>			return -1;

I thought about this again and here seems not to be consistent:
- even if nr reduces to 0, shouldn't avg_scan_cost be updated as well before return -1?
- if avg_scan_cost is not updated because nr is throttled, the first 
	time = cpu_clock(this);
  can be optimized. As nr is calculated and we already know which of the weight of cpumask and nr is greater.

Thanks,
-Aubrey