Date:   Wed, 7 Apr 2021 12:24:35 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v3] sched/fair: bring back select_idle_smt, but differently

On Wed, 7 Apr 2021 at 12:19, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Apr 07, 2021 at 11:54:37AM +0200, Peter Zijlstra wrote:
>
> > Let me have another poke at it.
>
> Pretty much what you did, except I also did s/smt/has_idle_core/ and
> fixed that @sd thing.
>
> Like so then?

Yes. Looks good to me

>
> ---
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6038,11 +6038,9 @@ static inline bool test_idle_cores(int c
>  {
>         struct sched_domain_shared *sds;
>
> -       if (static_branch_likely(&sched_smt_present)) {
> -               sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> -               if (sds)
> -                       return READ_ONCE(sds->has_idle_cores);
> -       }
> +       sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +       if (sds)
> +               return READ_ONCE(sds->has_idle_cores);
>
>         return def;
>  }
> @@ -6112,6 +6110,24 @@ static int select_idle_core(struct task_
>         return -1;
>  }
>
> +/*
> + * Scan the local SMT mask for idle CPUs.
> + */
> +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +       int cpu;
> +
> +       for_each_cpu(cpu, cpu_smt_mask(target)) {
> +               if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
> +                   !cpumask_test_cpu(cpu, sched_domain_span(sd)))
> +                       continue;
> +               if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> +                       return cpu;
> +       }
> +
> +       return -1;
> +}
> +
>  #else /* CONFIG_SCHED_SMT */
>
>  static inline void set_idle_cores(int cpu, int val)
> @@ -6128,6 +6144,11 @@ static inline int select_idle_core(struc
>         return __select_idle_cpu(core);
>  }
>
> +static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +       return -1;
> +}
> +
>  #endif /* CONFIG_SCHED_SMT */
>
>  /*
> @@ -6135,11 +6156,10 @@ static inline int select_idle_core(struc
>   * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
>   * average idle time for this rq (as found in rq->avg_idle).
>   */
> -static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
> +static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool has_idle_core, int target)
>  {
>         struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
>         int i, cpu, idle_cpu = -1, nr = INT_MAX;
> -       bool smt = test_idle_cores(target, false);
>         int this = smp_processor_id();
>         struct sched_domain *this_sd;
>         u64 time;
> @@ -6150,7 +6170,7 @@ static int select_idle_cpu(struct task_s
>
>         cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>
> -       if (sched_feat(SIS_PROP) && !smt) {
> +       if (sched_feat(SIS_PROP) && !has_idle_core) {
>                 u64 avg_cost, avg_idle, span_avg;
>
>                 /*
> @@ -6170,7 +6190,7 @@ static int select_idle_cpu(struct task_s
>         }
>
>         for_each_cpu_wrap(cpu, cpus, target) {
> -               if (smt) {
> +               if (has_idle_core) {
>                         i = select_idle_core(p, cpu, cpus, &idle_cpu);
>                         if ((unsigned int)i < nr_cpumask_bits)
>                                 return i;
> @@ -6184,10 +6204,10 @@ static int select_idle_cpu(struct task_s
>                 }
>         }
>
> -       if (smt)
> +       if (has_idle_core)
>                 set_idle_cores(this, false);
>
> -       if (sched_feat(SIS_PROP) && !smt) {
> +       if (sched_feat(SIS_PROP) && !has_idle_core) {
>                 time = cpu_clock(this) - time;
>                 update_avg(&this_sd->avg_scan_cost, time);
>         }
> @@ -6242,6 +6262,7 @@ static inline bool asym_fits_capacity(in
>   */
>  static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  {
> +       bool has_idle_core = false;
>         struct sched_domain *sd;
>         unsigned long task_util;
>         int i, recent_used_cpu;
> @@ -6321,7 +6342,17 @@ static int select_idle_sibling(struct ta
>         if (!sd)
>                 return target;
>
> -       i = select_idle_cpu(p, sd, target);
> +       if (static_branch_likely(&sched_smt_present)) {
> +               has_idle_core = test_idle_cores(target, false);
> +
> +               if (!has_idle_core && cpus_share_cache(prev, target)) {
> +                       i = select_idle_smt(p, sd, prev);
> +                       if ((unsigned int)i < nr_cpumask_bits)
> +                               return i;
> +               }
> +       }
> +
> +       i = select_idle_cpu(p, sd, has_idle_core, target);
>         if ((unsigned)i < nr_cpumask_bits)
>                 return i;
>

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