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Date:   Tue, 15 Dec 2020 15:45:16 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Yang Shi <shy828301@...il.com>, guro@...com, ktkhai@...tuozzo.com,
        shakeelb@...gle.com, mhocko@...e.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred

On Tue, Dec 15, 2020 at 01:22:33PM +1100, Dave Chinner wrote:
> On Mon, Dec 14, 2020 at 02:37:18PM -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 <shy828301@...il.com>
> > ---
> >  include/linux/memcontrol.h |   9 +++
> >  mm/memcontrol.c            | 110 ++++++++++++++++++++++++++++++++++++-
> >  mm/vmscan.c                |   4 ++
> >  3 files changed, 120 insertions(+), 3 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[];
> > +};
> 
> So you're effectively copy and pasting the memcg_shrinker_map
> infrastructure and doubling the number of allocations/frees required
> to set up/tear down a memcg? Why not add it to the struct
> memcg_shrinker_map like this:
> 
> struct memcg_shrinker_map {
>         struct rcu_head	rcu;
> 	unsigned long	*map;
> 	atomic_long_t	*nr_deferred;
> };
> 
> And when you dynamically allocate the structure, set the map and
> nr_deferred pointers to the correct offset in the allocated range.
> 
> Then this patch is really only changes to the size of the chunk
> being allocated, setting up the pointers and copying the relevant
> data from the old to new.

Fully agreed.

In the longer-term, it may be nice to further expand this and make
this the generalized intersection between cgroup, node and shrinkers.

There is large overlap with list_lru e.g. - with data of identical
scope and lifetime, but duplicative callbacks and management. If we
folded list_lru_memcg into the above data structure, we could also
generalize and reuse the existing callbacks.

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