linux-kernel - Re: [patch 05/15] sched: add a timer to handle CFS bandwidth refresh

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Message-ID: <BANLkTinRErBM_C=PB9BxH8QDVoNWJ_AdTA@mail.gmail.com>
Date:	Mon, 16 May 2011 05:56:56 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	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>, Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [patch 05/15] sched: add a timer to handle CFS bandwidth refresh

On Mon, May 16, 2011 at 3:18 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-05-03 at 02:28 -0700, Paul Turner wrote:
>> @@ -1003,6 +1003,8 @@ enqueue_entity(struct cfs_rq *cfs_rq, st
>>
>>         if (cfs_rq->nr_running == 1)
>>                 list_add_leaf_cfs_rq(cfs_rq);
>> +
>> +       start_cfs_bandwidth(cfs_rq);
>>  }
>>
>>  static void __clear_buddies_last(struct sched_entity *se)
>> @@ -1220,6 +1222,8 @@ static void put_prev_entity(struct cfs_r
>>                 update_stats_wait_start(cfs_rq, prev);
>>                 /* Put 'current' back into the tree. */
>>                 __enqueue_entity(cfs_rq, prev);
>> +
>> +               start_cfs_bandwidth(cfs_rq);
>>         }
>>         cfs_rq->curr = NULL;
>>  }
>
> OK, so while the first made sense the second had me go wtf?!, now I
> _think_ you do that because do_sched_cfs_period_timer() can return idle
> and stop the timer when no bandwidth consumption is seen for a while,
> and thus we need to re-start the timer when we put the entity to sleep,
> since that could have been a throttle.
>
> If that's so then neither really do make sense and a big fat comment is
> missing.
>
> So why not start on the same (but inverse) condition that makes it stop?
>

This was originally to guard the case that an entity was running on
stale (from a previous period) quota resulting in cfs_bandwidth->idle
and the timer not being re-instantiated.

Now that expiration is properly integrated I think the two cases are
analogous and that this can be dropped (and the start moved into the
(nr_running == 1) entity case on enqueue).

I think this is correct but my brain's a little fuzzy right now, will
confirm in the morning.
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