[Devel] [PATCH RHEL v2] mm: Reduce access frequency to shrinker_rwsem during shrink_slab

[Valeriy Vdovin]

Bug https://jira.sw.ru/browse/PSBM-99181 has introduced a problem: when
the kernel has opened NFS delegations and NFS server is not accessible
at the time when NFS shrinker is called, the whole shrinker list
execution gets stuck until NFS server is back. Being a problem in itself
it also introduces bigger problem - during that hang, the shrinker_rwsem
also gets locked, consequently no new mounts can be done at that time
because new superblock tries to register it's own shrinker and also gets
stuck at aquiring shrinker_rwsem.

Commit 9e9e35d050955648449498827deb2d43be0564e1 is a workaround for that
problem. It is known that during signle shrinker execution we do not
actually need to hold shrinker_rwsem so we release and reacqiure the
rwsem for each shrinker in the list.

Because of this workaround shrink_slab function now experiences a major
slowdown, because shrinker_rwsem gets accessed for each shrinker in the
list twice. On an idle fresh-booted system shrinker_list could be
iterated up to 1600 times a second, although originally the problem was
local to only one NFS shrinker.

This patch fixes commit 9e9e35d050955648449498827deb2d43be0564e1 in a
way that before calling for up_read for shrinker_rwsem, we check that
this is really an NFS shrinker by checking NFS magic in superblock, if
it is accessible from shrinker.

https://jira.sw.ru/browse/PSBM-99181

Co-authored-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
Signed-off-by: Valeriy Vdovin <valeriy.vdovin at virtuozzo.com>

Changes:
  v2: Added missing 'rwsem_is_contented' check
---
 fs/super.c  |  2 +-
 mm/vmscan.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++---------------
 2 files changed, 50 insertions(+), 17 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index f131d14..1cf377a 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -80,7 +80,7 @@ EXPORT_SYMBOL(dcache_is_low);
  * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
  * take a passive reference to the superblock to avoid this from occurring.
  */
-static unsigned long super_cache_scan(struct shrinker *shrink,
+unsigned long super_cache_scan(struct shrinker *shrink,
 				      struct shrink_control *sc)
 {
 	struct super_block *sb;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d7082d2..4fa86e7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -453,6 +453,20 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	return freed;
 }
 
+unsigned long super_cache_scan(struct shrinker *shrink,
+				      struct shrink_control *sc);
+
+static inline bool is_nfs_shrinker(struct shrinker *shrinker)
+{
+	struct super_block *sb = container_of(shrinker,
+		struct super_block, s_shrink);
+
+	if (shrinker->scan_objects == &super_cache_scan)
+		return sb->s_magic == NFS_SUPER_MAGIC;
+
+	return false;
+}
+
 struct shrinker *get_shrinker(struct shrinker *shrinker)
 {
 	/*
@@ -511,6 +525,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			.memcg = memcg,
 		};
 		struct shrinker *shrinker;
+		bool is_nfs;
 
 		shrinker = idr_find(&shrinker_idr, i);
 		if (unlikely(!shrinker)) {
@@ -518,6 +533,8 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			continue;
 		}
 
+		is_nfs = is_nfs_shrinker(shrinker);
+
 		/*
 		 * Take a refcnt on a shrinker so that it can't be freed or
 		 * removed from shrinker_idr (and shrinker_list). These way we
@@ -527,10 +544,16 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 		 * take too much time to finish (e.g. on nfs). And holding
 		 * global shrinker_rwsem can block registring and unregistring
 		 * of shrinkers.
+		 *
+		 * The up_read logic should only be executed for nfs shrinker
+		 * path, because it has proven to hang. For others it should be
+		 * skipped to reduce performance penalties.
 		 */
-		if(!get_shrinker(shrinker))
-			continue;
-		up_read(&shrinker_rwsem);
+		if (is_nfs) {
+			if (!get_shrinker(shrinker))
+				continue;
+			up_read(&shrinker_rwsem);
+		}
 
 		ret = do_shrink_slab(&sc, shrinker, priority);
 		if (ret == SHRINK_EMPTY) {
@@ -565,14 +588,18 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 		 * memcg_expand_one_shrinker_map if new shrinkers
 		 * were registred in the meanwhile.
 		 */
-		if (!down_read_trylock(&shrinker_rwsem)) {
-			freed = freed ? : 1;
+		if (is_nfs) {
+			if (!down_read_trylock(&shrinker_rwsem)) {
+				freed = freed ? : 1;
+				put_shrinker(shrinker);
+				return freed;
+			}
 			put_shrinker(shrinker);
-			return freed;
+			map = memcg_nid_shrinker_map(memcg, nid);
+			nr_max = min(shrinker_nr_max, map->nr_max);
+		} else if (rwsem_is_contended(&shrinker_rwsem)) {
+			break;
 		}
-		put_shrinker(shrinker);
-		map = memcg_nid_shrinker_map(memcg, nid);
-		nr_max = min(shrinker_nr_max, map->nr_max);
 	}
 unlock:
 	up_read(&shrinker_rwsem);
@@ -648,13 +675,17 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 			.for_drop_caches = for_drop_caches,
 		};
 
+		bool is_nfs = is_nfs_shrinker(shrinker);
+
 		if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
 			sc.nid = 0;
 
 		/* See comment in shrink_slab_memcg. */
-		if(!get_shrinker(shrinker))
-			continue;
-		up_read(&shrinker_rwsem);
+		if (is_nfs) {
+			if (!get_shrinker(shrinker))
+				continue;
+			up_read(&shrinker_rwsem);
+		}
 
 		ret = do_shrink_slab(&sc, shrinker, priority);
 		if (ret == SHRINK_EMPTY)
@@ -665,12 +696,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 		 * We can safely continue traverse of the shrinker_list as
 		 * our shrinker is still on the list due to refcount.
 		 */
-		if (!down_read_trylock(&shrinker_rwsem)) {
-			freed = freed ? : 1;
+		if (is_nfs) {
+			if (!down_read_trylock(&shrinker_rwsem)) {
+				freed = freed ? : 1;
+				put_shrinker(shrinker);
+				goto out;
+			}
 			put_shrinker(shrinker);
-			goto out;
 		}
-		put_shrinker(shrinker);
 	}
 
 	up_read(&shrinker_rwsem);
-- 
1.8.3.1