* Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> [2018-05-09 09:41:48]: > On Mon, May 07, 2018 at 04:06:07AM -0700, Srikar Dronamraju wrote: > > > @@ -1876,7 +1877,18 @@ static void numa_migrate_preferred(struct task_struct *p) > > > > > > /* Periodically retry migrating the task to the preferred node */ > > > interval = min(interval, msecs_to_jiffies(p->numa_scan_period) / 16); > > > - p->numa_migrate_retry = jiffies + interval; > > > + numa_migrate_retry = jiffies + interval; > > > + > > > + /* > > > + * Check that the new retry threshold is after the current one. If > > > + * the retry is in the future, it implies that wake_affine has > > > + * temporarily asked NUMA balancing to backoff from placement. > > > + */ > > > + if (numa_migrate_retry > p->numa_migrate_retry) > > > + return; > > > > The above check looks wrong. This check will most likely to be true, > > numa_migrate_preferred() itself is called either when jiffies > > > p->numa_migrate_retry or if the task's numa_preferred_nid has changed. > > > > You're right, without affine wakeups with a wakeup-intensive workload > the path may never be hit and with the current code, it effectively acts > as a broken throttling mechanism. I haven't tried on an x86 box, but still trying to get my head around that check. How does affine wakeups differ for this check. Lets say p->numa_migrate_retry was set by wake_affine and task has crossed that temporary period where we dont want the task to undergo numa balancing. Now the task is back at numa_migrate_preferred(); p->numa_migrate_retry is lesser than jiffies (something like "current jiffies - 100"). It would always return back from that check. In the other scenario, where wake_affine set p->numa_migrate_preferred to a bigger value, the task calls numa_migrate_preferred(), numa_migrate_preferred could be before p->numa_migrate_preferred. In such a case, we should have stopped the task from migration. However we overwrite p->numa_migrate_preferred and do the task_numa_migrate(). Somehow this doesn't seem to achieve what the commit intended. Or did I misunderstand? -- Thanks and Regards Srikar -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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