From 07252b0f97150b03050f74f42250f275566d70b9 Mon Sep 17 00:00:00 2001 From: Qi Zheng Date: Fri, 9 Jun 2023 08:15:13 +0000 Subject: [PATCH] Revert "mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()" This reverts commit 1643db98d9b314e0a592d152603094fbf7ab906e. 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. So we still need shrinker_rwsem in synchronize_shrinkers() after reverting the shrinker_srcu related changes. [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/20230609081518.3039120-3-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 | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 4730dba..0ba0e11 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -831,11 +831,15 @@ EXPORT_SYMBOL(unregister_shrinker); /** * synchronize_shrinkers - Wait for all running shrinkers to complete. * - * This is useful to guarantee that all shrinker invocations have seen an - * update, before freeing memory. + * This is equivalent to calling unregister_shrink() and register_shrinker(), + * but atomically and with less overhead. This is useful to guarantee that all + * shrinker invocations have seen an update, before freeing memory, similar to + * rcu. */ void synchronize_shrinkers(void) { + down_write(&shrinker_rwsem); + up_write(&shrinker_rwsem); atomic_inc(&shrinker_srcu_generation); synchronize_srcu(&shrinker_srcu); } -- 2.7.4