On Tue, Nov 29, 2022 at 9:40 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Mina Almasry <almasrymina@xxxxxxxxxx> 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 > > > > I may be missing something but as far I can tell reclaim is disabled > > for allocations from lower tier memory: > > https://elixir.bootlin.com/linux/v6.1-rc7/source/mm/vmscan.c#L1583 > > #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM) > > We have GFP_NOWAIT set in gfp. > Ah, thanks. I missed that. > > I think this is maybe a good thing when doing proactive demotion. In > > this case we probably don't want to try to reclaim from lower tier > > nodes and instead fail the proactive demotion. > > Do you have some real use cases for this? If so, we can tweak the > logic. > Nothing real at the moment. I was thinking this may be something desirable to tune at some point. > > However I can see this being desirable when the top tier nodes are > > under real memory pressure to deflect that pressure to the lower tier > > nodes. > > Yes. > > Best Regards, > Huang, Ying > > >> 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. > >> > >> 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? > >> > >> 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. > > > > I sent RFC v2 patch: > > https://lore.kernel.org/linux-mm/20221130020328.1009347-1-almasrymina@xxxxxxxxxx/T/#u > > > > Please take a look when convenient. Thanks! > > > >> > > >> > >> Best Regards, > >> Huang, Ying > >> >
