linux-kernel - Re: [PATCH 2/8] sched/fair: Add more sched_asym

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Date:   Tue, 9 Feb 2021 09:42:00 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>
Subject: Re: [PATCH 2/8] sched/fair: Add more sched_asym_cpucapacity static
 branch checks

On Thu, 28 Jan 2021 at 19:32, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> Rik noted a while back that a handful of
>
>   sd->flags & SD_ASYM_CPUCAPACITY
>
> & family in the CFS load-balancer code aren't guarded by the
> sched_asym_cpucapacity static branch.
>
> The load-balancer is already doing a humongous amount of work, but turning
> those checks into NOPs for those who don't need it is fairly
> straightforward, so do that.

It would be good to get figures showing an improvement. We are there
in the LB which is a slow path. Whereas using the static branch in
fast path makes sense, I don't see the benefit there. Also why not
doing the same for others like SD_ASYM_PACKING.
I'm not convinced by this change which IMO doesn't provide any benefit
but adds complexity.

>
> Suggested-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/fair.c  | 11 ++++++-----
>  kernel/sched/sched.h |  6 ++++++
>  2 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0f6a4e58ce3c..7d01fa0bfc7e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8465,7 +8465,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>                         continue;
>
>                 /* Check for a misfit task on the cpu */
> -               if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> +               if (sd_has_asym_cpucapacity(env->sd) &&
>                     sgs->group_misfit_task_load < rq->misfit_task_load) {
>                         sgs->group_misfit_task_load = rq->misfit_task_load;
>                         *sg_status |= SG_OVERLOAD;
> @@ -8522,7 +8522,8 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>          * CPUs in the group should either be possible to resolve
>          * internally or be covered by avg_load imbalance (eventually).
>          */
> -       if (sgs->group_type == group_misfit_task &&
> +       if (static_branch_unlikely(&sched_asym_cpucapacity) &&
> +           sgs->group_type == group_misfit_task &&
>             (!group_smaller_max_cpu_capacity(sg, sds->local) ||
>              sds->local_stat.group_type != group_has_spare))
>                 return false;
> @@ -8605,7 +8606,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>          * throughput. Maximize throughput, power/energy consequences are not
>          * considered.
>          */
> -       if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
> +       if (sd_has_asym_cpucapacity(env->sd) &&
>             (sgs->group_type <= group_fully_busy) &&
>             (group_smaller_min_cpu_capacity(sds->local, sg)))
>                 return false;
> @@ -8728,7 +8729,7 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
>         }
>
>         /* Check if task fits in the group */
> -       if (sd->flags & SD_ASYM_CPUCAPACITY &&
> +       if (sd_has_asym_cpucapacity(sd) &&
>             !task_fits_capacity(p, group->sgc->max_capacity)) {
>                 sgs->group_misfit_task_load = 1;
>         }
> @@ -9419,7 +9420,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>                  * Higher per-CPU capacity is considered better than balancing
>                  * average load.
>                  */
> -               if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> +               if (sd_has_asym_cpucapacity(env->sd) &&
>                     capacity_of(env->dst_cpu) < capacity &&
>                     nr_running == 1)
>                         continue;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 045b01064c1e..21bd71f58c06 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1482,6 +1482,12 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
>  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
>  extern struct static_key_false sched_asym_cpucapacity;
>
> +static inline bool sd_has_asym_cpucapacity(struct sched_domain *sd)
> +{
> +       return static_branch_unlikely(&sched_asym_cpucapacity) &&
> +               sd->flags & SD_ASYM_CPUCAPACITY;
> +}
> +
>  struct sched_group_capacity {
>         atomic_t                ref;
>         /*
> --
> 2.27.0
>