Re: [RFC][PATCH 2/5] sched/fair: Make select_idle_cpu() proportional to cores

[Vincent Guittot]

On Mon, 14 Dec 2020 at 18:07, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Instead of calculating how many (logical) CPUs to scan, compute how
> many cores to scan.
>
> This changes behaviour for anything !SMT2.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c |   19 ++++++++++++++-----
>  kernel/sched/fair.c |   12 ++++++++++--
>  2 files changed, 24 insertions(+), 7 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7454,11 +7454,20 @@ int sched_cpu_activate(unsigned int cpu)
>         balance_push_set(cpu, false);
>
>  #ifdef CONFIG_SCHED_SMT
> -       /*
> -        * When going up, increment the number of cores with SMT present.
> -        */
> -       if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> -               static_branch_inc_cpuslocked(&sched_smt_present);
> +       do {
> +               int weight = cpumask_weight(cpu_smt_mask(cpu));
> +               extern int sched_smt_weight;
> +
> +               if (weight > sched_smt_weight)
> +                       sched_smt_weight = weight;
> +
> +               /*
> +                * When going up, increment the number of cores with SMT present.
> +                */
> +               if (weight == 2)
> +                       static_branch_inc_cpuslocked(&sched_smt_present);
> +
> +       } while (0);
>  #endif
>         set_cpu_active(cpu, true);
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6010,6 +6010,8 @@ static inline int find_idlest_cpu(struct
>  DEFINE_STATIC_KEY_FALSE(sched_smt_present);
>  EXPORT_SYMBOL_GPL(sched_smt_present);
>
> +int sched_smt_weight = 1;
> +
>  static inline void set_idle_cores(int cpu, int val)
>  {
>         struct sched_domain_shared *sds;
> @@ -6124,6 +6126,8 @@ static int select_idle_smt(struct task_s
>
>  #else /* CONFIG_SCHED_SMT */
>
> +#define sched_smt_weight       1
> +
>  static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
>  {
>         return -1;
> @@ -6136,6 +6140,8 @@ static inline int select_idle_smt(struct
>
>  #endif /* CONFIG_SCHED_SMT */
>
> +#define sis_min_cores          2
> +
>  /*
>   * Scan the LLC domain for idle CPUs; this is dynamically regulated by
>   * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
> @@ -6166,10 +6172,12 @@ static int select_idle_cpu(struct task_s
>                 avg_cost = this_sd->avg_scan_cost + 1;
>
>                 span_avg = sd->span_weight * avg_idle;
> -               if (span_avg > 4*avg_cost)
> +               if (span_avg > sis_min_cores * avg_cost)
>                         nr = div_u64(span_avg, avg_cost);
>                 else
> -                       nr = 4;
> +                       nr = sis_min_cores;
> +
> +               nr *= sched_smt_weight;

My comment apply of the final result of the patchset where
select_idle_cpu/cpore are merged
IIUC, sched_smt_weight is the max number of CPUs per core but we
already loop all CPUs of a core in one loop so we don't need to loop
more than the number of core.

>
>                 time = cpu_clock(this);
>         }
>
>