Messages in this thread | | | Date | Thu, 22 May 2014 09:47:58 -0400 | From | Tejun Heo <> | Subject | Re: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue_work() |
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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 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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