linux-kernel - Re: [RFC v5 4/6] sched/fair: Tune task wake-up logic to pack small background tasks on fewer cores

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Date:   Wed, 9 Oct 2019 13:34:06 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Parth Shah <parth@...ux.ibm.com>
Cc:     Hillf Danton <hdanton@...a.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Valentin Schneider <valentin.schneider@....com>,
        Pavel Machek <pavel@....cz>,
        Doug Smythies <dsmythies@...us.net>,
        Quentin Perret <qperret@...rret.net>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [RFC v5 4/6] sched/fair: Tune task wake-up logic to pack small
 background tasks on fewer cores

On Wed, 9 Oct 2019 at 11:23, Parth Shah <parth@...ux.ibm.com> wrote:
>
>
>
> On 10/8/19 6:58 PM, Hillf Danton wrote:
> >
> > On Mon,  7 Oct 2019 14:00:49 +0530 Parth Shah wrote:
> >> +/*
> >> + * Try to find a non idle core in the system  based on few heuristics:
> >> + * - Keep track of overutilized (>80% util) and busy (>12.5% util) CPUs
> >> + * - If none CPUs are busy then do not select the core for task packing
> >> + * - If atleast one CPU is busy then do task packing unless overutilized CPUs
> >> + *   count is < busy/2 CPU count
> >> + * - Always select idle CPU for task packing
> >> + */
> >> +static int select_non_idle_core(struct task_struct *p, int prev_cpu, int target)
> >> +{
> >> +    struct cpumask *cpus = this_cpu_cpumask_var_ptr(turbo_sched_mask);
> >> +    int iter_cpu, sibling;
> >> +
> >> +    cpumask_and(cpus, cpu_online_mask, p->cpus_ptr);
> >> +
> >> +    for_each_cpu_wrap(iter_cpu, cpus, prev_cpu) {
> >> +            int idle_cpu_count = 0, non_idle_cpu_count = 0;
> >> +            int overutil_cpu_count = 0;
> >> +            int busy_cpu_count = 0;
> >> +            int best_cpu = iter_cpu;
> >> +
> >> +            for_each_cpu(sibling, cpu_smt_mask(iter_cpu)) {
> >> +                    __cpumask_clear_cpu(sibling, cpus);
> >> +                    if (idle_cpu(iter_cpu)) {
> >
> > Would you please elaborate the reasons that the iter cpu is checked idle
> > more than once for finding a busy core?
> >
>
> Thanks for looking at the patches.
> Could you please point me out where iter_cpu is checked more than once?

I think that point is that you have a sibling that there is
for_each_cpu(sibling, cpu_smt_mask(iter_cpu) but you never use sibling
in the loop except for clearing it on the cpumask cpus
All the tests are done with iter_cpu so you will test several time
iter_cpus but never the other sibling
Should you use sibling instead ?


>
> >> +                            idle_cpu_count++;
> >> +                            best_cpu = iter_cpu;
> >> +                    } else {
> >> +                            non_idle_cpu_count++;
> >> +                            if (cpu_overutilized(iter_cpu))
> >> +                                    overutil_cpu_count++;
> >> +                            if (is_cpu_busy(cpu_util(iter_cpu)))
> >> +                                    busy_cpu_count++;
> >> +                    }
> >> +            }
> >> +
> >
>