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? > 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
