linux-kernel - Re: [PATCH 4/7] sched: implement force_cpus

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Message-ID: <1260268453.3935.1106.camel@laptop>
Date:	Tue, 08 Dec 2009 11:34:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	tglx@...utronix.de, mingo@...e.hu, avi@...hat.com, efault@....de,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	Gautham R Shenoy <ego@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4/7] sched: implement force_cpus_allowed()

On Tue, 2009-12-08 at 18:12 +0900, Tejun Heo wrote:
> > So its only needed in order to flush a workqueue from CPU_DOWN_PREPARE?
> > And all you need it to place a new kthread on a !active cpu?
> 
> Yes, that's all I need. 

Then you don't need most of that patch, you don't need to touch the
migration stuff since a new kthread isn't running.

All you need to do is make a kthread_bind/set_cpus_allowed variant that
checks against cpu_online_mask instead of cpu_active_mask.

It might even make sense to have kthread_bind() always check with
cpu_online_mask as the kernel really ought to know what its doing
anyway.

You also don't need to play trickery with PF_THREAD_BOUND, since a new
kthread will not have that set.

In fact, your patch is against a tree that still has cpu_online_mask in
all those places, so you wouldn't have needed any of that, confused.. ?!

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