Re: [PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred

[Roman Gushchin <guro@fb.com>]

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 <shy828301@gmail.com>
> ---
>  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?