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SubjectRe: [PATCH] sched: Fast idling of CPU when system is partially loaded
On Thu, Jun 12, 2014 at 02:25:59PM -0700, Tim Chen wrote:
> @@ -2630,7 +2630,7 @@ static inline struct task_struct *
> pick_next_task(struct rq *rq, struct task_struct *prev)
> {
> const struct sched_class *class = &fair_sched_class;
> - struct task_struct *p;
> + struct task_struct *p = NULL;
>
> /*
> * Optimization: we know that if all tasks are in
> @@ -2638,9 +2638,13 @@ pick_next_task(struct rq *rq, struct task_struct *prev)
> */
> if (likely(prev->sched_class == class &&
> rq->nr_running == rq->cfs.h_nr_running)) {
> - p = fair_sched_class.pick_next_task(rq, prev);
> - if (unlikely(p == RETRY_TASK))
> - goto again;
> +
> + /* If no cpu has more than 1 task, skip */
> + if (rq->nr_running > 0 || rq->rd->overload) {
> + p = fair_sched_class.pick_next_task(rq, prev);
> + if (unlikely(p == RETRY_TASK))
> + goto again;
> + }
>
> /* assumes fair_sched_class->next == idle_sched_class */
> if (unlikely(!p))


Please move this into pick_next_task_fair(). You're slowing down the
important fast path of picking a task when there actually is something
to do.

Also, its a layering violation -- the idle balance things you're trying
to avoid is a fair_sched_class affair.

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9855e87..00ab38c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5863,7 +5863,8 @@ static inline int sg_capacity(struct lb_env *env, struct sched_group *group)
> */
> static inline void update_sg_lb_stats(struct lb_env *env,
> struct sched_group *group, int load_idx,
> - int local_group, struct sg_lb_stats *sgs)
> + int local_group, struct sg_lb_stats *sgs,
> + bool *overload)
> {
> unsigned long load;
> int i;
> @@ -5881,6 +5882,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>
> sgs->group_load += load;
> sgs->sum_nr_running += rq->nr_running;
> + if (overload && rq->nr_running > 1)
> + *overload = true;
> #ifdef CONFIG_NUMA_BALANCING
> sgs->nr_numa_running += rq->nr_numa_running;
> sgs->nr_preferred_running += rq->nr_preferred_running;
> @@ -5991,6 +5994,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> struct sched_group *sg = env->sd->groups;
> struct sg_lb_stats tmp_sgs;
> int load_idx, prefer_sibling = 0;
> + bool overload = false;
>
> if (child && child->flags & SD_PREFER_SIBLING)
> prefer_sibling = 1;
> @@ -6011,7 +6015,13 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> update_group_power(env->sd, env->dst_cpu);
> }
>
> - update_sg_lb_stats(env, sg, load_idx, local_group, sgs);
> + if (env->sd->parent)
> + update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> + NULL);
> + else
> + /* gather overload info if we are at root domain */
> + update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> + &overload);
>
> if (local_group)
> goto next_group;
> @@ -6045,6 +6055,15 @@ next_group:
>
> if (env->sd->flags & SD_NUMA)
> env->fbq_type = fbq_classify_group(&sds->busiest_stat);
> +
> + if (!env->sd->parent) {
> + /* update overload indicator if we are at root domain */
> + int i = cpumask_first(sched_domain_span(env->sd));
> + struct rq *rq = cpu_rq(i);
> + if (rq->rd->overload != overload)
> + rq->rd->overload = overload;
> + }
> +
> }
>
> /**

The worry I have is that this update is 'slow'. We could have grown many
tasks since the last update.


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