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

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Message-ID: <AANLkTinRS=pi3fQSy63KZY0XZ6xcOtZtscV0RKusCz0L@mail.gmail.com>
Date:	Wed, 2 Mar 2011 00:05:51 -0800
From:	Paul Turner <pjt@...gle.com>
To:	bharata@...ux.vnet.ibm.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

On Tue, Mar 1, 2011 at 11:23 PM, Bharata B Rao
<bharata@...ux.vnet.ibm.com> wrote:
> 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 ?
>

As I recall this was for entity reweight

In this case on_rq is cached and it's possible we'll tip over into a
throttled state when we update_curr().  I think with reweight_entity()
this may no longer be required.

In my current v5 stack I've actually reworked the throttling logic to make
update_curr() non-state changing again and thus eliminated the
associated checks and cases.

> Regards,
> Bharata.
>
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