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

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Message-ID: <4B1E378A.5050101@kernel.org>
Date:	Tue, 08 Dec 2009 20:24:58 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.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()

Hello,

On 12/08/2009 07:34 PM, Peter Zijlstra wrote:
> 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. 

Let me augment the above sentence.  Yes, that's all I need *during
CPU_DOWN*.  During CPU_UP, I need to migrate back left running workers
which survived from the last CPU_DOWN.  In the original patch, the
down path is worker_maybe_bind_and_lock() and the latter path is
trustee_unset_rogue().

> 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.

kthread_bind() doesn't check against cpu_online_mask.  Isn't
set_cpus_allowed() variant which checks against cpu_online_mask is
what's implemented by the patch (+ PF_THREAD_BOUND bypass)?

> 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.

Oh... yeah, it was a bit strange that the function doesn't check
against cpu onliness but if that is removed what would be the point of
kthread_bind() when set_cpus_allowed() provides pretty much the same
capability?

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

Yeap, but when the cpu comes back online, those kthreads need to be
rebound to the cpu.

> 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.. ?!

To migrate back the workers from CPU_ONLINE callback.

Thanks.

-- 
tejun
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