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SubjectRe: [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@arm.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@surriel.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.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
>

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