On 05/22/2014 10:21 PM, Lai Jiangshan wrote: > 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
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 >> -- >> 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/ > -- 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/
