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

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Date:	Mon, 16 May 2011 12:18:08 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Turner <pjt@...gle.com>
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 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?
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