From c534f7cca6b9b1c0dc97d6e9c5587858d4330cd9 Mon Sep 17 00:00:00 2001 From: Qi Zheng Date: Fri, 9 Jun 2023 08:15:14 +0000 Subject: [PATCH] Revert "mm: vmscan: hold write lock to reparent shrinker nr_deferred" This reverts commit b3cabea3c9153fd42fe5cb851ac58b51ea2b32b8. Kernel test robot reports -88.8% regression in stress-ng.ramfs.ops_per_sec test case [1], which is caused by commit f95bdb700bc6 ("mm: vmscan: make global slab shrink lockless"). The root cause is that SRCU has to be careful to not frequently check for SRCU read-side critical section exits. Therefore, even if no one is currently in the SRCU read-side critical section, synchronize_srcu() cannot return quickly. That's why unregister_shrinker() has become slower. We will try to use the refcount+RCU method [2] proposed by Dave Chinner to continue to re-implement the lockless slab shrink. Because there will be other readers after reverting the shrinker_srcu related changes, so it is better to restore to hold read lock to reparent shrinker nr_deferred. [1]. https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@intel.com/ [2]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@dread.disaster.area/ Link: https://lkml.kernel.org/r/20230609081518.3039120-4-qi.zheng@linux.dev Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@intel.com Signed-off-by: Qi Zheng Cc: Dave Chinner Cc: Kirill Tkhai Cc: Muchun Song Cc: Roman Gushchin Cc: Vlastimil Babka Signed-off-by: Andrew Morton --- mm/vmscan.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 0ba0e11..d1d309f 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -433,7 +433,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg) parent = root_mem_cgroup; /* Prevent from concurrent shrinker_info expand */ - down_write(&shrinker_rwsem); + down_read(&shrinker_rwsem); for_each_node(nid) { child_info = shrinker_info_protected(memcg, nid); parent_info = shrinker_info_protected(parent, nid); @@ -442,7 +442,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg) atomic_long_add(nr, &parent_info->nr_deferred[i]); } } - up_write(&shrinker_rwsem); + up_read(&shrinker_rwsem); } static bool cgroup_reclaim(struct scan_control *sc) -- 2.7.4