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. > > 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
