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SubjectRe: [PATCH] sched: Fast idling of CPU when system is partially loaded
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On Thu, 2014-06-12 at 14:25 -0700, Tim Chen wrote:

> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
> kernel/sched/core.c | 12 ++++++++----
> kernel/sched/fair.c | 23 +++++++++++++++++++++--
> kernel/sched/sched.h | 10 ++++++++--
> 3 files changed, 37 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c6b9879..4f57221 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -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) {

Hi Tim,

If it is skipping if no cpu has more than 1 task, should the
above have the additional check for (rq->nr_running > 1) instead
of (rq->nr_running > 0)?

> + 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))
> 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);

Would it make the code cleaner if we always call:

+ update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
&overload);

and in update_sg_lb_stats():

+ bool is_root_domain = (env->sd->parent == NULL)


+ /* gather overload info if we are at root domain */
+ if (is_root_domain && rq->nr_running > 1)
+ *overload = true;

> 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);

Perhaps we could just use:

struct rq *rq = env->dst_rq;

> + if (rq->rd->overload != overload)
> + rq->rd->overload = overload;
> + }
> +
> }
>
> /**
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e47679b..a0cd5c1 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -477,6 +477,9 @@ struct root_domain {
> cpumask_var_t span;
> cpumask_var_t online;
>
> + /* Indicate more than one runnable task for any CPU */
> + bool overload;
> +
> /*
> * The bit corresponding to a CPU gets set here if such CPU has more
> * than one runnable -deadline task (as it is below for RT tasks).
> @@ -1212,15 +1215,18 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
>
> rq->nr_running = prev_nr + count;
>
> -#ifdef CONFIG_NO_HZ_FULL
> if (prev_nr < 2 && rq->nr_running >= 2) {
> + if (!rq->rd->overload)
> + rq->rd->overload = true;
> +
> +#ifdef CONFIG_NO_HZ_FULL
> if (tick_nohz_full_cpu(rq->cpu)) {
> /* Order rq->nr_running write against the IPI */
> smp_wmb();
> smp_send_reschedule(rq->cpu);
> }
> - }
> #endif
> + }
> }
>
> static inline void sub_nr_running(struct rq *rq, unsigned count)




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