Re: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue_work()

[Lai Jiangshan]

On Thu, May 22, 2014 at 9:47 PM, Tejun Heo <tj@xxxxxxxxxx> 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

> 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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