linux-kernel - Re: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue

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Date:	Mon, 26 May 2014 11:19:05 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue_work()

On 05/22/2014 10:21 PM, Lai Jiangshan wrote:
> On Thu, May 22, 2014 at 9:47 PM, Tejun Heo <tj@...nel.org> wrote:
>> On Thu, May 22, 2014 at 04:44:16PM +0800, Lai Jiangshan wrote:
>>> When pwq->refcnt == 0, the retrying is guaranteed to make forward-progress.
>>> The comment above the code explains it well:
>>>
>>>       /*
>>>        * pwq is determined and locked.  For unbound pools, we could have
>>>        * raced with pwq release and it could already be dead.  If its
>>>        * refcnt is zero, repeat pwq selection.  Note that pwqs never die
>>>        * without another pwq replacing it in the numa_pwq_tbl or while
>>>        * work items are executing on it, so the retrying is guaranteed to
>>>        * make forward-progress.
>>>        */
>>>
>>> It means the cpu_relax() here is useless and sometimes misleading,
>>> it should retry directly and make some progress rather than waste time.
>>
>> cpu_relax() doesn't have much to do with guaranteeing forward
>> progress.  It's about giving a breather during busy wait so that the
> 
> This is not busy wait, the retry and numa_pwq_tbl() guarantee that
> the retry will get a new pwq (even without cpu_relax()) as the comments says,
> and the refcnt of this new pwq is very very likely non-zero and
> cpu_relax() can't
> increase the probability of non-zero-refcnt. cpu_relax() is useless here.
> 
> It is different from spin_lock() or some other spin code.
> 
> it is similar to the loop of __task_rq_lock() which also guarantees progress.
> 
> Thanks,
> Lai

Ping.

Any comments?

> 
>> waiting cpu doesn't busy loop claiming the same cache lines over and
>> over ultimately delaying the event being waited on.  If you're doing a
>> busy wait, you better use cpu_relax().
>>
>> Thanks.
>>
>> --
>> tejun
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