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

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Message-ID: <20140611131536.GB21191@linux.vnet.ibm.com>
Date:	Wed, 11 Jun 2014 18:45:36 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Kirill Tkhai <tkhai@...dex.ru>
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()

> > * 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.
>

I think you misread my comment.

Currently migrate_task() gets called from migration_call() and in the
migration_call() before migrate_tasks(), set_rq_offline() should put
tasks back using unthrottle_cfs_rq().

So my question is: Why are these tasks not getting unthrottled
through we are calling set_rq_offline? To me set_rq_offline is
calling the actual sched class routines to do the needful.

I can understand about deadline tasks, because we don't have a deadline
But thats the only tasks that we need to fix.

> >>  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.

What I meant was only if rq->rd isn't set, then we don't call
set_rq_offline, which seems very reasonable.
--
Thanks and Regards
Srikar Dronamraju

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