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Subject: Re: [patch 04/15] sched: throttle cfs_rq entities which exceed
 their local quota
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Paul Turner <pjt@google.com>
Cc: linux-kernel@vger.kernel.org, Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Dhaval Giani <dhaval.giani@gmail.com>,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Srivatsa Vaddagiri <vatsa@in.ibm.com>,
        Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@elte.hu>, Pavel Emelyanov <xemul@openvz.org>,
        Nikhil Rao <ncrao@google.com>
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	 <20110323030449.047028257@google.com>
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Date: Tue, 05 Apr 2011 15:28:20 +0200
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On Tue, 2011-03-22 at 20:03 -0700, Paul Turner wrote:

> @@ -1249,6 +1257,9 @@ entity_tick(struct cfs_rq *cfs_rq, struc
>  	 */
>  	update_curr(cfs_rq);
>  
> +	/* check that entity's usage is still within quota (if enabled) */
> +	check_cfs_rq_quota(cfs_rq);
> +
>  	/*
>  	 * Update share accounting for long-running entities.
>  	 */

You already have a hook in update_curr() to account quota, why not also
use that to trigger the reschedule? request_cfs_rq_quota() already has
the information we failed to replenish the local quota.

Then when you've gotten rid of check_cfs_rq_quota() there isn't a second
user of within_bandwidth() and you can fold:

> @@ -1230,6 +1233,9 @@ static void put_prev_entity(struct cfs_r
>  	if (prev->on_rq)
>  		update_curr(cfs_rq);
>  
> +	if (!within_bandwidth(cfs_rq))
> +		throttle_cfs_rq(cfs_rq);
> +
>  	check_spread(cfs_rq, prev);
>  	if (prev->on_rq) {
>  		update_stats_wait_start(cfs_rq, prev);

Into a single hook. 

> @@ -1447,10 +1544,15 @@ static void dequeue_task_fair(struct rq 
>         for_each_sched_entity(se) {
>                 cfs_rq = cfs_rq_of(se);
>                 dequeue_entity(cfs_rq, se, flags);
> -
> +               /* end evaluation on throttled cfs_rq */
> +               if (cfs_rq_throttled(cfs_rq)) {
> +                       se = NULL;
> +                       break;
> +               }
>                 /* Don't dequeue parent if it has other entities besides us */
>                 if (cfs_rq->load.weight)
>                         break;
> +               check_cfs_rq_quota(cfs_rq);
>                 flags |= DEQUEUE_SLEEP;
>         }

dequeue_entity() calls update_curr(), so again, by folding
check_cfs_rq_quota() into your update_curr() hook this becomes simpler.

> +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
> +{
> +       struct task_group *tg;
> +       struct sched_entity *se;
> +
> +       if (cfs_rq_throttled(cfs_rq))
> +               return 1;
> +
> +       tg = cfs_rq->tg;
> +       se = tg->se[cpu_of(rq_of(cfs_rq))];
> +       if (!se)
> +               return 0;
> +
> +       for_each_sched_entity(se) {
> +               if (cfs_rq_throttled(cfs_rq_of(se)))
> +                       return 1;
> +       }
> +
> +       return 0;
> +}

You can actually call for_each_sched_entity() with se==NULL, saves a few lines.