Re: [PATCH] sched: Fast idling of CPU when system is partially loaded

[Jason Low <jason.low2@hp.com>]

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)