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 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. 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. > 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 >
