linux-kernel - Re: [PATCH 04/40] sched: implement __set_cpus

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Message-ID: <4B56C047.6070808@kernel.org>
Date:	Wed, 20 Jan 2010 17:35:19 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	torvalds@...ux-foundation.org, mingo@...e.hu, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 04/40] sched: implement __set_cpus_allowed()

Hello,

On 01/19/2010 05:37 PM, Peter Zijlstra wrote:
> On Tue, 2010-01-19 at 10:07 +0900, Tejun Heo wrote:
>>
>> It's also necessary to guarantee forward progress during CPU_DOWN.
>> The problem with kthread_bind() is that it's not synchronized against
>> CPU hotplug operations.  It needs outer synchronization like calling
>> it directly from CPU_DOWN_PREP.  I guess it's doable but I think it
>> would be better to simply share the backend implementation between
>> set_cpus_allowed_ptr() and kthread_bind().
> 
> OK, so you're saying you need to migrate the rescue thread during
> cpu-down. That thread is guaranteed sleeping right,

No, it's not.  It might have been tasked to process works from other
CPUs.

> if it were not it'd not be elegible to run on our dying cpu.  Hence
> kthread_bind() ought to just work, no?

Why wouldn't it be elegible?

Commit e2912009fb7b715728311b0d8fe327a1432b3f79 killed the ability to
bind a kthread to a dead CPU which means that the only differences
between kthread_bind() and set_cpus_allowed_ptr() are...

* Whether to use cpu_online_mask or cpu_active_mask.

* Whether to set or check PF_THREAD_BOUND.

Wouldn't it make more sense to share the backend implementation
between kthread_bind() and set_cpus_allowed_ptr() instead of making
kthread_bind() a special case?  The goals of the two functions are
basically identical.  Why have two separate implementations?
kthread_bind() implementation as it currently stands is pretty fragile
too.  Making kthread_bind() backed by set_cpus_allowed_ptr() will make
it more robust and less error-prone and all that's necessary to
achieve that is modifying sanity checks.

Thanks.

-- 
tejun
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