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charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Dec 3, 2020 at 12:07 PM Roman Gushchin wrote: > > On Thu, Dec 03, 2020 at 10:03:44AM -0800, Yang Shi wrote: > > On Wed, Dec 2, 2020 at 8:54 PM Yang Shi wrote: > > > > > > On Wed, Dec 2, 2020 at 7:06 PM Roman Gushchin wrote: > > > > > > > > On Wed, Dec 02, 2020 at 10:27:21AM -0800, Yang Shi wrote: > > > > > Currently the number of deferred objects are per shrinker, but some slabs, for example, > > > > > vfs inode/dentry cache are per memcg, this would result in poor isolation among memcgs. > > > > > > > > > > The deferred objects typically are generated by __GFP_NOFS allocations, one memcg with > > > > > excessive __GFP_NOFS allocations may blow up deferred objects, then other innocent memcgs > > > > > may suffer from over shrink, excessive reclaim latency, etc. > > > > > > > > > > For example, two workloads run in memcgA and memcgB respectively, workload in B is vfs > > > > > heavy workload. Workload in A generates excessive deferred objects, then B's vfs cache > > > > > might be hit heavily (drop half of caches) by B's limit reclaim or global reclaim. > > > > > > > > > > We observed this hit in our production environment which was running vfs heavy workload > > > > > shown as the below tracing log: > > > > > > > > > > <...>-409454 [016] .... 28286961.747146: mm_shrink_slab_start: super_cache_scan+0x0/0x1a0 ffff9a83046f3458: > > > > > nid: 1 objects to shrink 3641681686040 gfp_flags GFP_HIGHUSER_MOVABLE|__GFP_ZERO pgs_scanned 1 lru_pgs 15721 > > > > > cache items 246404277 delta 31345 total_scan 123202138 > > > > > <...>-409454 [022] .... 28287105.928018: mm_shrink_slab_end: super_cache_scan+0x0/0x1a0 ffff9a83046f3458: > > > > > nid: 1 unused scan count 3641681686040 new scan count 3641798379189 total_scan 602 > > > > > last shrinker return val 123186855 > > > > > > > > > > The vfs cache and page cache ration was 10:1 on this machine, and half of caches were dropped. > > > > > This also resulted in significant amount of page caches were dropped due to inodes eviction. > > > > > > > > > > Make nr_deferred per memcg for memcg aware shrinkers would solve the unfairness and bring > > > > > better isolation. > > > > > > > > > > When memcg is not enabled (!CONFIG_MEMCG or memcg disabled), the shrinker's nr_deferred > > > > > would be used. And non memcg aware shrinkers use shrinker's nr_deferred all the time. > > > > > > > > > > Signed-off-by: Yang Shi > > > > > --- > > > > > include/linux/memcontrol.h | 9 +++ > > > > > mm/memcontrol.c | 112 ++++++++++++++++++++++++++++++++++++- > > > > > mm/vmscan.c | 4 ++ > > > > > 3 files changed, 123 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > > > > index 922a7f600465..1b343b268359 100644 > > > > > --- a/include/linux/memcontrol.h > > > > > +++ b/include/linux/memcontrol.h > > > > > @@ -92,6 +92,13 @@ struct lruvec_stat { > > > > > long count[NR_VM_NODE_STAT_ITEMS]; > > > > > }; > > > > > > > > > > + > > > > > +/* Shrinker::id indexed nr_deferred of memcg-aware shrinkers. */ > > > > > +struct memcg_shrinker_deferred { > > > > > + struct rcu_head rcu; > > > > > + atomic_long_t nr_deferred[]; > > > > > +}; > > > > > > > > The idea makes total sense to me. But I wonder if we can add nr_deferred to > > > > struct list_lru_one, instead of adding another per-memcg per-shrinker entity? > > > > I guess it can simplify the code quite a lot. What do you think? > > > > > > Aha, actually this exactly was what I did at the first place. But Dave > > > NAK'ed this approach. You can find the discussion at: > > > https://lore.kernel.org/linux-mm/20200930073152.GH12096@dread.disaster.area/. > > Yes, this makes sense for me. Thank you for the link! > > > > > I did prototypes for both approaches (move nr_deferred to list_lru or > > to memcg). I preferred the list_lru approach at the first place. But > > Dave's opinion does make perfect sense to me. So I dropped that > > list_lru one. That email elaborated why moving nr_deferred to list_lru > > is not appropriate. > > Hm, shouldn't we move list_lru to memcg then? It's not directly related > to your patchset, but maybe it's something we should consider in the future. I haven't thought about this yet. I agree we could look into it further later on. > > What worries me is that with your patchset we'll have 3 separate > per-memcg (per-node) per-shrinker entity, each with slightly different > approach to allocate/extend/reparent/release. So it begs for some > unification. I don't think it's a showstopper for your work though, it > can be done later. Off the top of my head, we may be able to have shrinker_info struct, it should look like: struct shrinker_info { atomic_long_t nr_deferred; /* Just one bit is used now */ u8 map:1; } struct memcg_shrinker_info { struct rcu_head rcu; /* Indexed by shrinker ID */ struct shrinker_info info[]; } Then in struct mem_cgroup_per_node, we could have: struct mem_cgroup_per_node { .... struct memcg_shrinker_info __rcu *shrinker_info; .... } In this way shrinker_info should be allocated to all memcgs, including root. But shrinker could ignore root's map bit. We may waste a little bit memory, but we get unification. Would that work?