Re: [RFC][PATCH] sched: Avoid select_idle_sibling() forwake_affine(.sync=true)

[Mike Galbraith]

On Thu, 2013-09-26 at 11:58 +0200, Peter Zijlstra wrote: 
> On Wed, Sep 25, 2013 at 10:56:17AM +0200, Mike Galbraith wrote:
> > That will make pipe-test go fugly -> pretty, and help very fast/light
> > localhost network, but eat heavier localhost overlap recovery.  We need
> > a working (and cheap) overlap detector scheme, so we can know when there
> > is enough to be worth going after.
> 
> We used to have an overlap detectoring thing.. It went away though.

Yup.. guilty as charged your honor.

> But see if you can make something like the below work?

It'll have the same, preemption etc., problems as the whacked.  Could
bring that back and have another go at making it actually work, maybe
it'll be good enough to cause more gain than pain.

> You could make it a general overlap thing and try without the sync too I
> suppose.. 
> 
> ---
>  include/linux/sched.h |  3 +++
>  kernel/sched/fair.c   | 25 +++++++++++++++++++++++++
>  2 files changed, 28 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index b5344de..5428016 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -974,6 +974,9 @@ struct sched_entity {
>  	u64			vruntime;
>  	u64			prev_sum_exec_runtime;
>  
> +	u64			last_sync_wakeup;
> +	u64			avg_overlap;
> +
>  	u64			nr_migrations;
>  
>  #ifdef CONFIG_SCHEDSTATS
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2b89cd2..47b0d0f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2913,6 +2913,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	struct sched_entity *se = &p->se;
>  	int task_sleep = flags & DEQUEUE_SLEEP;
>  
> +	if (se->last_sync_wakeup) {
> +		u64 overlap;
> +		s64 diff;
> +	       
> +		overlap = rq->clock - se->last_sync_wakeup;
> +		se->last_sync_wakeup = 0;
> +
> +		diff = overlap - se->avg_overlap;
> +		se->avg_overlap += diff >> 8;
> +	}
> +
>  	for_each_sched_entity(se) {
>  		cfs_rq = cfs_rq_of(se);
>  		dequeue_entity(cfs_rq, se, flags);
> @@ -3429,6 +3440,9 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  	int want_affine = 0;
>  	int sync = wake_flags & WF_SYNC;
>  
> +	if (sync)
> +		p->se.last_sync_wakeup = sched_clock_cpu(cpu);
> +
>  	if (p->nr_cpus_allowed == 1)
>  		return prev_cpu;
>  
> @@ -3461,6 +3475,17 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
>  			prev_cpu = cpu;
>  
> +		/*
> +		 * Don't bother with select_idle_sibling() in the case of a sync wakeup
> +		 * where we know the only running task will soon go-away. Going
> +		 * through select_idle_sibling will only lead to pointless ping-pong.
> +		 */
> +		if (sync && prev_cpu == cpu && cpu_rq(cpu)->nr_running == 1 &&
> +		    current->se.avg_overlap < 10000) {
> +			new_cpu = cpu;
> +			goto unlock;
> +		}
> +
>  		new_cpu = select_idle_sibling(p, prev_cpu);
>  		goto unlock;
>  	}


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