Hi Aaron, I fear my blood-brain barrier doesn't let much of mm/ code in, so I'm adding a few interested people in Cc. Meanwhile a few comments below: On Thu, Feb 03, 2022 at 09:43:39PM +0000, Aaron Tomlin wrote: > Hi Frederic, > > If I understand correctly, in the context of the idle task and a nohz_full > CPU, quiet_vmstat() can be called: before stopping the idle tick, entering > an idle state and on exit. In particular, for the latter case, when the > idle task is required to reschedule, the idle tick can remain stopped and > the timer expiration time endless i.e., KTIME_MAX. Now, indeed before a > nohz_full CPU enters an idle state, CPU-specific vmstat counters should > be processed to ensure the respective values have been reset and folded > into the zone specific vm_stat[]. That being said, it can only occur when: > the idle tick was previously stopped, and reprogramming of the timer is not > required. 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? > > 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? Thanks!
