linux-kernel - Re: [RFC][PATCH 06/22] sched: SCHED_DEADLINE handles spacial kthreads

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Date:	Thu, 11 Nov 2010 17:32:43 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Raistlin <raistlin@...ux.it>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chris Friesen <cfriesen@...tel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Claudio Scordino <claudio@...dence.eu.com>,
	michael trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Luca Abeni <luca.abeni@...tn.it>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 06/22] sched: SCHED_DEADLINE handles spacial
	kthreads

On 11/11, Peter Zijlstra wrote:
>
> On Thu, 2010-11-11 at 16:27 +0100, Oleg Nesterov wrote:
> >
> > Sorry, can't resist, off-topic quiestion. Do we really need
> > migration_call()->migrate_live_tasks() ?
> >
> > With the recent changes, try_to_wake_up() can never choose
> > the dead (!cpu_online) cpu if the task was deactivated.
> >
> > Looks like we should only worry about the running tasks, and
> > migrate_dead_tasks()->pick_next_task() loop seems to all work
> > we need.
> >
> > (Of course, we can't just remove migrate_live_tasks(), at leat
> >  migrate_dead() needs simple changes).
> >
> > What do you think?
>
> Yes, I think we can make that work, we could even move that
> migrate_live_tasks() into CPU_DYING, which is before this point.

Hmm, I think you are right. In this case we can also simplify
migrate-from-dead-cpu paths, we know that nobody can touch rq->lock,
every CPU runs stop_machine_cpu_stop() with irqs disabled. No need
to get/put task_struct, etc.

Oleg.

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