Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes: > On Wed, Jul 19, 2023 at 01:59:00PM +0800, Huang, Ying wrote: >> > The big remaaining corner case to watch out for is where the sum >> > of the boosted pcp->high exceeds the low watermark. If that should ever >> > happen then potentially a premature OOM happens because the watermarks >> > are fine so no reclaim is active but no pages are available. It may even >> > be the case that the sum of pcp->high should not exceed *min* as that >> > corner case means that processes may prematurely enter direct reclaim >> > (not as bad as OOM but still bad). >> >> Sorry, I don't understand this. When pages are moved from buddy to PCP, >> zone NR_FREE_PAGES will be decreased in rmqueue_bulk(). That is, pages >> in PCP will be counted as used instead of free. And, in >> zone_watermark_ok*() and zone_watermark_fast(), zone NR_FREE_PAGES is >> used to check watermark. So, if my understanding were correct, if the >> number of pages in PCP is larger than low/min watermark, we can still >> trigger reclaim. Whether is my understanding correct? >> > > You're right, I didn't check the timing of the accounting and all that > occurred to me was "the timing of when watermarks trigger kswapd or > direct reclaim may change as a result of PCP adaptive resizing". Even > though I got the timing wrong, the shape of the problem just changes. > I suspect that excessively large PCP high relative to the watermarks may > mean that reclaim happens prematurely if too many pages are pinned by PCP > pages as the zone free pages approaches the watermark. Yes. I think so too. In addition to reclaim, falling back to remote NUMA node may happen prematurely too. > While disabling the adaptive resizing during reclaim will limit the > worst of the problem, it may still be the case that kswapd is woken > early simply because there are enough CPUs pinning pages in PCP > lists. Similarly, depending on the size of pcp->high and the gap > between the watermarks, it's possible for direct reclaim to happen > prematurely. I could still be wrong because I'm not thinking the > problem through fully, examining the code or thinking about the > implementation. It's simply worth keeping in mind the impact elevated > PCP high values has on the timing of watermarks failing. If it's > complex enough, it may be necessary to have a separate patch dealing > with the impact of elevated pcp->high on watermarks. Sure. I will keep this in mind. We may need to check zone watermark when tuning pcp->high and free some pages from PCP before falling back to other node or reclaiming. -- Best Regards, Huang, Ying
