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). > 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 >
