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 -- 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/
