On Thu 11-01-24 16:08:57, Jianfeng Wang wrote: > > > On 1/11/24 1:54 PM, Andrew Morton wrote: > > On Thu, 11 Jan 2024 10:54:45 -0800 Jianfeng Wang <jianfeng.w.wang@xxxxxxxxxx> wrote: > > > >> > >>> Unless you can show any actual runtime effect of this patch then I think > >>> it shouldn't be merged. > >>> > >> > >> Thanks for raising your concern. > >> I'd call it a trade-off rather than "not really correct". Look at > >> unmap_region() / free_pages_and_swap_cache() written by Linus. These are in > >> favor of this pattern, which indicates that the trade-off (i.e. draining > >> local CPU or draining all CPUs or no draining at all) had been made in the > >> same way in the past. I don't have a specific runtime effect to provide, > >> except that it will free 10s kB pages immediately during OOM. You are missing an important point. Those two calls are quite different. oom_reaper unmaps memory after all the reclaim attempts have failed. That includes draining all sorts of caches on the way. Including draining LRU pcp cache (look for lru_add_drain_all in the reclaim path). > > I don't think it's necessary to run lru_add_drain() for each vma. Once > > we've done it it once, it can be skipped for additional vmas. > > > Agreed. > > > That's pretty minor because the second and successive calls will be > > cheap. But it becomes much more significant if we switch to > > lru_add_drain_all(), which sounds like what we should be doing here. > > Is it possible? > > > What do you both think of adding lru_add_drain_all() prior to the for loop? lru_add_drain_all relies on WQs. And we absolutely do not want to get oom_reaper stuck just because all the WQ is jammed. So no, this is actually actively harmful! All that being said I stand by my previous statement that this patch is not doing anything measurably useful. Prove me wrong otherwise I am against merging "just for consistency patch". Really, we should go and re-evaluate existing local lru draining callers. I wouldn't be surprised if we removed some of them. -- Michal Hocko SUSE Labs
