Re: [v2 PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred

Dave Chinner
2020-12-15T02:22:39Z

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@gmail.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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com