linux-kernel - Re: [CFS Bandwidth Control v4 3/7] sched: throttle cfs_rq entities which exceed their local quota

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Date:	Wed, 2 Mar 2011 12:53:59 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Paul Turner <pjt@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Nikhil Rao <ncrao@...gle.com>
Subject: Re: [CFS Bandwidth Control v4 3/7] sched: throttle cfs_rq entities
 which exceed their local quota

Hi Paul,

On Tue, Feb 15, 2011 at 07:18:34PM -0800, Paul Turner wrote:
> @@ -1015,6 +1046,14 @@ enqueue_entity(struct cfs_rq *cfs_rq, st
>  	 * Update run-time statistics of the 'current'.
>  	 */
>  	update_curr(cfs_rq);
> +
> +
> +#ifdef CONFIG_CFS_BANDWIDTH
> +	if (!entity_is_task(se) && (cfs_rq_throttled(group_cfs_rq(se)) ||
> +	     !group_cfs_rq(se)->nr_running))
> +		return;
> +#endif
> +
>  	update_cfs_load(cfs_rq, 0);
>  	account_entity_enqueue(cfs_rq, se);
>  	update_cfs_shares(cfs_rq);

> @@ -1363,6 +1407,9 @@ enqueue_task_fair(struct rq *rq, struct 
>  			break;
>  		cfs_rq = cfs_rq_of(se);
>  		enqueue_entity(cfs_rq, se, flags);
> +		/* don't continue to enqueue if our parent is throttled */
> +		if (cfs_rq_throttled(cfs_rq))
> +			break;

1. This check (in enqueue_task_fair) ensures that if the cfs_rq we just enqueued
se to is throttled, we bail our from futher enqueueing of the hierarchy.

2. In enqueue_entity() we check if the entity we are enqueing owns a throttled
hieararchy and refuse to enqueue if true. And we silently refuse but continue
with futher enqueue attempts.

I see that 1 can happen when there a task belonging to a throttled group
wakes up.

Can you pls explain the scenario when 2 is needed ?

Regards,
Bharata.
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