linux-kernel - Re: [PATCH 1/2] sched: Rework migrate

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Date:	Wed, 11 Jun 2014 16:20:54 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Tkhai Kirill <ktkhai@...allels.com>
Subject: Re: [PATCH 1/2] sched: Rework migrate_tasks()

11.06.2014, 15:24, "Srikar Dronamraju" <srikar@...ux.vnet.ibm.com>:
> * Kirill Tkhai <ktkhai@...allels.com> [2014-06-11 13:52:10]:
>>  Currently migrate_tasks() skips throttled tasks,
>>  because they are not pickable by pick_next_task().
>
> Before migrate_tasks() is called, we do call set_rq_offline(), in
> migration_call().
>
> Shouldnt this take care of unthrottling the tasks and making sure that
> they can be picked by pick_next_task().

If we do this separate for every class, we'll have to do this 3 times.
Furthermore, deadline class does not have a list of throttled tasks.
So we'll have to the same as I did: to lock tasklist_lock and to iterate
throw all of the tasks in the system just to found deadline tasks.

>>  These tasks stay on dead cpu even after they
>>  becomes unthrottled. They are not schedulable
>>  till user manually changes their affinity or till
>>  cpu becomes alive again.
>
> If we are still seeing tasks not being picked by pick_next_task(), then
> can it probably mean that rq->rd was NULL?

Unthrottle functions dl_task_timer() and unthrottle_cfs_rq() put tasks and
queues back. They do not look at rq->rd.

do_sched_rt_period_timer() does not unthrottle dead queue at all (they
are excluded from rd).

Thanks,
Kirill
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