On Fri 07-01-22 17:19:28, Yu Zhao wrote: > On Fri, Jan 07, 2022 at 10:06:15AM +0100, Michal Hocko wrote: > > On Tue 04-01-22 13:22:24, Yu Zhao wrote: > > > To exploit spatial locality, the aging prefers to walk page tables to > > > search for young PTEs. And this patch paves the way for that. > > > > > > An mm_struct list is maintained for each memcg, and an mm_struct > > > follows its owner task to the new memcg when this task is migrated. > > > > How does this work actually for the memcg reclaim? I can see you > > lru_gen_migrate_mm on the task migration. My concern is, though, that > > such a task leaves all the memory behind in the previous memcg (in > > cgroup v2, in v1 you can opt in for charge migration). If you move the > > mm to a new memcg then you age it somewhere where the memory is not > > really consumed. > > There are two options to gather the accessed bit: page table walks and > rmap walks. Page table walks sweep dense hotspots that are NOT > misplaced in terms of reclaim scope (lruvec); rmap walks cover what > page table walks miss, e.g., misplaced dense hotspots or sparse ones. > > Dense hotspots are stored in Bloom filters for each lruvec. > > If an mm leaves everything in the old memcg, page table walks in the > new memcg reclaim path basically ignore this mm after the first scan, > because everything is misplaced. OK, so do I get it right that pages mapped from a different memcg than the reclaimed one are considered effectivelly non-present from the the reclaim logic POV? This would be worth mentioning in the migration callback because it is not really that straightforward to put those two together. > In the old memcg reclaim path, page table walks won't see this mm > at all. But rmap walks will catch everything later in the eviction > path, i.e., lru_gen_look_around(). This function is less efficient > compared with page table walks because, for each rmap walk of a > non-shared page, it only can gather the accessed bit from 64 PTEs at > most. But it's still a lot faster than the original rmap, which only > gathers the accessed bit from a single PTE, for each walk of a > non-shared page. Again, something that should be really documented. -- Michal Hocko SUSE Labs
