linux-kernel - Re: [PATCH] sched/fair: Optimize select_idle

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Message-ID: <6d188305-66ab-81cf-6340-34d155dcaf3b@arm.com>
Date:   Fri, 13 Dec 2019 11:28:00 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     "chengjian (D)" <cj.chengjian@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        chenwandun@...wei.com, xiexiuqi@...wei.com, liwei391@...wei.com,
        huawei.libin@...wei.com, bobo.shaobowang@...wei.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org
Subject: Re: [PATCH] sched/fair: Optimize select_idle_cpu

On 13/12/2019 09:57, chengjian (D) wrote:
> 
> in select_idle_smt()
> 
> /*
>  * Scan the local SMT mask for idle CPUs.
>  */
> static int select_idle_smt(struct task_struct *p, int target)
> {
>     int cpu, si_cpu = -1;
> 
>     if (!static_branch_likely(&sched_smt_present))
>         return -1;
> 
>     for_each_cpu(cpu, cpu_smt_mask(target)) {
>         if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>             continue;
>         if (available_idle_cpu(cpu))
>             return cpu;
>         if (si_cpu == -1 && sched_idle_cpu(cpu))
>             si_cpu = cpu;
>     }
> 
>     return si_cpu;
> }
> 
> 
> Why don't we do the same thing in this function,
> 
> although cpu_smt_present () often has few CPUs.
> 
> it is better to determine the 'p->cpus_ptr' first.
> 
> 

Like you said the gains here would probably be small - the highest SMT
count I'm aware of is SMT8 (POWER9). Still, if we end up with both
select_idle_core() and select_idle_cpu() using that pattern, it would make
sense IMO to align select_idle_smt() with those.

> 
> 
>