Re: [PATCH v6 5/9] mm: multigenerational lru: mm_struct list

[Michal Hocko]

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