linux-kernel - Re: [patch 08/15] sched: migrate throttled tasks on HOTPLUG

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Date:	Tue, 5 Apr 2011 19:31:17 -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 08/15] sched: migrate throttled tasks on HOTPLUG

On Tue, Apr 5, 2011 at 6:28 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-03-22 at 20:03 -0700, Paul Turner wrote:
>> Throttled tasks are invisisble to cpu-offline since they are not eligible for
>> selection by pick_next_task().  The regular 'escape' path for a thread that is
>> blocked at offline is via ttwu->select_task_rq, however this will not handle a
>> throttled group since there are no individual thread wakeups on an unthrottle.
>>
>> Resolve this by unthrottling offline cpus so that threads can be migrated.
>
> Fair enough, the flip side is that they all can again increase their
> debt by a whole tick, right?
>

In the case where they enqueue on to a new rq and we can't catch the
bw condition until entity_tick I suppose it's possible to grab up to
an extra tick.

This is actually analogous to a task waking up when other cpus have
drained quota -- this case can be properly handled by an explicit
check/throttle in the enqueue_task_fair() path in the non-on-cpu case.
 Will add.
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