Re: [PATCH v5 3/5] sched/fair: Rework sched_use_asym_prio() and sched_asym_prefer()

[Vincent Guittot <vincent.guittot@linaro.org>]

On Sat, 10 Feb 2024 at 12:36, <alexs@kernel.org> wrote:
>
> From: Alex Shi <alexs@kernel.org>
>
> sched_use_asym_prio() and sched_asym_prefer() are used together in various
> places. Consolidate them into a single function sched_asym().
>
> The existing sched_group_asym() is only used when collecting statistics

nit: The existing sched_asym()

> of a scheduling group. Rename it as sched_group_asym(), and remove the
> obsolete function description.
>
> This makes the code easier to read. No functional changes.
>
> Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Signed-off-by: Alex Shi <alexs@kernel.org>
> Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Reviewed-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/fair.c | 45 ++++++++++++++++++++-------------------------
>  1 file changed, 20 insertions(+), 25 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 607dc310b355..426eda9eda57 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9746,8 +9746,18 @@ static bool sched_use_asym_prio(struct sched_domain *sd, int cpu)
>         return sd->flags & SD_SHARE_CPUCAPACITY || is_core_idle(cpu);
>  }
>
> +static inline bool sched_asym(struct sched_domain *sd, int dst_cpu, int src_cpu)
> +{
> +       /*
> +        * First check if @dst_cpu can do asym_packing load balance. Only do it
> +        * if it has higher priority than @src_cpu.
> +        */
> +       return sched_use_asym_prio(sd, dst_cpu) &&
> +               sched_asym_prefer(dst_cpu, src_cpu);
> +}
> +
>  /**
> - * sched_asym - Check if the destination CPU can do asym_packing load balance
> + * sched_group_asym - Check if the destination CPU can do asym_packing balance
>   * @env:       The load balancing environment
>   * @sgs:       Load-balancing statistics of the candidate busiest group
>   * @group:     The candidate busiest group
> @@ -9755,34 +9765,21 @@ static bool sched_use_asym_prio(struct sched_domain *sd, int cpu)
>   * @env::dst_cpu can do asym_packing if it has higher priority than the
>   * preferred CPU of @group.
>   *
> - * SMT is a special case. If we are balancing load between cores, @env::dst_cpu
> - * can do asym_packing balance only if all its SMT siblings are idle. Also, it
> - * can only do it if @group is an SMT group and has exactly on busy CPU. Larger
> - * imbalances in the number of CPUS are dealt with in find_busiest_group().
> - *
> - * If we are balancing load within an SMT core, or at PKG domain level, always
> - * proceed.
> - *
>   * Return: true if @env::dst_cpu can do with asym_packing load balance. False
>   * otherwise.
>   */
>  static inline bool
> -sched_asym(struct lb_env *env, struct sg_lb_stats *sgs, struct sched_group *group)
> +sched_group_asym(struct lb_env *env, struct sg_lb_stats *sgs, struct sched_group *group)
>  {
> -       /* Ensure that the whole local core is idle, if applicable. */
> -       if (!sched_use_asym_prio(env->sd, env->dst_cpu))
> -               return false;
> -
>         /*
> -        * CPU priorities does not make sense for SMT cores with more than one
> +        * CPU priorities do not make sense for SMT cores with more than one
>          * busy sibling.
>          */
> -       if (group->flags & SD_SHARE_CPUCAPACITY) {
> -               if (sgs->group_weight - sgs->idle_cpus != 1)
> -                       return false;
> -       }
> +       if ((group->flags & SD_SHARE_CPUCAPACITY) &&
> +           (sgs->group_weight - sgs->idle_cpus != 1))
> +               return false;
>
> -       return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
> +       return sched_asym(env->sd, env->dst_cpu, group->asym_prefer_cpu);
>  }
>
>  /* One group has more than one SMT CPU while the other group does not */
> @@ -9938,7 +9935,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>         /* Check if dst CPU is idle and preferred to this group */
>         if (!local_group && env->sd->flags & SD_ASYM_PACKING &&
>             env->idle != CPU_NOT_IDLE && sgs->sum_h_nr_running &&
> -           sched_asym(env, sgs, group)) {
> +           sched_group_asym(env, sgs, group)) {
>                 sgs->group_asym_packing = 1;
>         }
>
> @@ -11037,8 +11034,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>                  * SMT cores with more than one busy sibling.
>                  */
>                 if ((env->sd->flags & SD_ASYM_PACKING) &&
> -                   sched_use_asym_prio(env->sd, i) &&
> -                   sched_asym_prefer(i, env->dst_cpu) &&
> +                   sched_asym(env->sd, i, env->dst_cpu) &&
>                     nr_running == 1)
>                         continue;
>
> @@ -11908,8 +11904,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                  * preferred CPU must be idle.
>                  */
>                 for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
> -                       if (sched_use_asym_prio(sd, i) &&
> -                           sched_asym_prefer(i, cpu)) {
> +                       if (sched_asym(sd, i, cpu)) {
>                                 flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>                                 goto unlock;
>                         }
> --
> 2.43.0
>