On Thu 2022-02-17 13:47 +0100, Frederic Weisbecker wrote: > So, to make sure I understand, the issue is that with nohz_full, we may > well enter into the idle loop with the tick already stopped. We may also > exit from idle without restarting the tick (again only with nohz_full). And > so this can cause the vmstat to not be flushed upon idle entry. Right? Hi Frederic, Yes - this is exactly it. > > A customer provided some evidence which indicates that the idle tick was > > stopped; albeit, CPU-specific vmstat counters still remained populated. > > Thus one can only assume quiet_vmstat() was not invoked on return to the > > idle loop. > > > > Unfortunately, I suspect this divergence might erroneously prevent a > > reclaim attempt by kswapd. If the number of zone specific free pages are > > below their per-cpu drift value then zone_page_state_snapshot() is used to > > compute a more accurate view of the aforementioned statistic. > > Thus any task blocked on the NUMA node specific pfmemalloc_wait queue will > > be unable to make significant progress via direct reclaim unless it is > > killed after being woken up by kswapd (see throttle_direct_reclaim()). > > That being said, eventually reclaim should give up if the conditions are > > correct, no? > Now if quiet_vmstat() isn't called, the vmstat_work should fix this later, > right? Or does that happen too late perhaps? If I understand correctly, in the context of nohz_full, since such work is deferred, it will only be handled in a scenario when the periodic/or scheduling-clock tick is enabled i.e. the timer was reprogrammed on exit from idle. Kind regards, -- Aaron Tomlin
