Mina Almasry <almasrymina@xxxxxxxxxx> writes: > On Tue, Nov 29, 2022 at 7:56 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Johannes Weiner <hannes@xxxxxxxxxxx> writes: >> >> > Hello Ying, >> > >> > On Thu, Nov 24, 2022 at 01:51:20PM +0800, Huang, Ying wrote: >> >> 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. >> >> >> >> 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. >> > >> > If they're hotter than reclaim candidates on the toptier, shouldn't >> > they get promoted instead and make room that way? We may have to tweak >> > the watermark logic a bit to facilitate that (allow promotions where >> > regular allocations already fail?). But this sort of resorting would >> > be preferable to age inversions. >> >> Now it's legal to enable demotion and disable promotion. Yes, this is >> wrong configuration in general. But should we trigger OOM for these >> users? >> >> And now promotion only works for default NUMA policy (and MPOL_BIND to >> both promotion source and target nodes with MPOL_F_NUMA_BALANCING). If >> we use some other NUMA policy, the pages cannot be promoted too. >> >> > The mlock scenario sounds possible. In that case, it wouldn't be an >> > aging inversion, since there is nothing colder on the CXL node. >> > >> > Maybe a bypass check should explicitly consult the demotion target >> > watermarks against its evictable pages (similar to the file_is_tiny >> > check in prepare_scan_count)? >> >> Yes. This sounds doable. >> >> > Because in any other scenario, if there is a bug in the promo/demo >> > coordination, I think we'd rather have the OOM than deal with age >> > inversions causing intermittent performance issues that are incredibly >> > hard to track down. >> >> Previously, I thought that people will always prefer performance >> regression than OOM. Apparently, I am wrong. >> >> Anyway, I think that we need to reduce the possibility of OOM or falling >> back to reclaim as much as possible firstly. Do you agree? >> > > I've been discussing this with a few folks here. I think FWIW general > feeling here is that demoting from top tier nodes is preferred, except > in extreme circumstances we would indeed like to run with a > performance issue rather than OOM a customer VM. I wonder if there is > another way to debug mis-tiered pages rather than trigger an oom to > debug. > > One thing I think/hope we can trivially agree on is that proactive > reclaim/demotion is _not_ an extreme circumstance. I would like me or > someone from the team to follow up with a patch that disables fallback > to reclaim on proactive reclaim/demotion (sc->proactive). Yes. This makes sense to me. Best Regards, Huang, Ying >> One possibility, can we fall back to reclaim only if the sc->priority is >> small enough (even 0)? >> > > This makes sense to me. > >> Best Regards, >> Huang, Ying >>
