On Thu, May 22, 2014 at 9:47 PM, Tejun Heo <t...@kernel.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 > 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
