Yang Shi <shy828301@xxxxxxxxx> writes: > On Tue, Nov 29, 2022 at 9:33 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Yang Shi <shy828301@xxxxxxxxx> writes: >> >> > On Mon, Nov 28, 2022 at 4:54 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> Yang Shi <shy828301@xxxxxxxxx> writes: >> >> >> >> > On Wed, Nov 23, 2022 at 9:52 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> >> >> Hi, Johannes, >> >> >> >> >> >> Johannes Weiner <hannes@xxxxxxxxxxx> writes: >> >> >> [...] >> >> >> > >> >> >> > The fallback to reclaim actually strikes me as wrong. >> >> >> > >> >> >> > Think of reclaim as 'demoting' the pages to the storage tier. If we >> >> >> > have a RAM -> CXL -> storage hierarchy, we should demote from RAM to >> >> >> > CXL and from CXL to storage. If we reclaim a page from RAM, it means >> >> >> > we 'demote' it directly from RAM to storage, bypassing potentially a >> >> >> > huge amount of pages colder than it in CXL. That doesn't seem right. >> >> >> > >> >> >> > If demotion fails, IMO it shouldn't satisfy the reclaim request by >> >> >> > breaking the layering. Rather it should deflect that pressure to the >> >> >> > lower layers to make room. This makes sure we maintain an aging >> >> >> > pipeline that honors the memory tier hierarchy. >> >> >> >> >> >> Yes. I think that we should avoid to fall back to reclaim as much as >> >> >> possible too. Now, when we allocate memory for demotion >> >> >> (alloc_demote_page()), __GFP_KSWAPD_RECLAIM is used. So, we will trigger >> >> >> kswapd reclaim on lower tier node to free some memory to avoid fall back >> >> >> to reclaim on current (higher tier) node. This may be not good enough, >> >> >> for example, the following patch from Hasan may help via waking up >> >> >> kswapd earlier. >> >> > >> >> > For the ideal case, I do agree with Johannes to demote the page tier >> >> > by tier rather than reclaiming them from the higher tiers. But I also >> >> > agree with your premature OOM concern. >> >> > >> >> >> >> >> >> https://lore.kernel.org/linux-mm/b45b9bf7cd3e21bca61d82dcd1eb692cd32c122c.1637778851.git.hasanalmaruf@xxxxxx/ >> >> >> >> >> >> Do you know what is the next step plan for this patch? >> >> >> >> >> >> Should we do even more? >> >> > >> >> > In my initial implementation I implemented a simple throttle logic >> >> > when the demotion is not going to succeed if the demotion target has >> >> > not enough free memory (just check the watermark) to make migration >> >> > succeed without doing any reclamation. Shall we resurrect that? >> >> >> >> Can you share the link to your throttle patch? Or paste it here? >> > >> > I just found this on the mailing list. >> > https://lore.kernel.org/linux-mm/1560468577-101178-8-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx/ >> >> Per my understanding, this patch will avoid demoting if there's no free >> space on demotion target? If so, I think that we should trigger kswapd >> reclaiming on demotion target before that. And we can simply avoid to >> fall back to reclaim firstly, then avoid to scan as an improvement as >> that in your patch above. > > Yes, it should. The rough idea looks like: > > if (the demote target is contended) > wake up kswapd > reclaim_throttle(VMSCAN_THROTTLE_DEMOTION) > retry demotion > > The kswapd is responsible for clearing the contention flag. We may do this, at least for demotion in kswapd. But I think that this could be the second step optimization after we make correct choice between demotion/reclaim. What if the pages in demotion target is too hot to be reclaimed first? Should we reclaim in fast memory node to avoid OOM? Best Regards, Huang, Ying >> >> > But it didn't have the throttling logic, I may not submit that version >> > to the mailing list since we decided to drop this and merge mine and >> > Dave's. >> > >> > Anyway it is not hard to add the throttling logic, we already have a >> > few throttling cases in vmscan, for example, "mm/vmscan: throttle >> > reclaim until some writeback completes if congested". >> >> >> >> > Waking kswapd sooner is fine to me, but it may be not enough, for >> >> > example, the kswapd may not keep up so remature OOM may happen on >> >> > higher tiers or reclaim may still happen. I think throttling the >> >> > reclaimer/demoter until kswapd makes progress could avoid both. And >> >> > since the lower tiers memory typically is quite larger than the higher >> >> > tiers, so the throttle should happen very rarely IMHO. >> >> > >> >> >> >> >> >> From another point of view, I still think that we can use falling back >> >> >> to reclaim as the last resort to avoid OOM in some special situations, >> >> >> for example, most pages in the lowest tier node are mlock() or too hot >> >> >> to be reclaimed. >> >> >> >> >> >> > So I'm hesitant to design cgroup controls around the current behavior. >> >> >> >> Best Regards, >> >> Huang, Ying
