The patch titled
Subject: Revert "mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()"
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
revert-mm-vmscan-remove-shrinker_rwsem-from-synchronize_shrinkers.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/revert-mm-vmscan-remove-shrinker_rwsem-from-synchronize_shrinkers.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Subject: Revert "mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()"
Date: Fri, 9 Jun 2023 08:15:13 +0000
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@xxxxxxxxx/
[2]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@xxxxxxxxxxxxxxxxxxx/
Link: https://lkml.kernel.org/r/20230609081518.3039120-3-qi.zheng@xxxxxxxxx
Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@xxxxxxxxx
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Kirill Tkhai <tkhai@xxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
--- a/mm/vmscan.c~revert-mm-vmscan-remove-shrinker_rwsem-from-synchronize_shrinkers
+++ a/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);
}
_
Patches currently in -mm which might be from zhengqi.arch@xxxxxxxxxxxxx are
revert-mm-shrinkers-convert-shrinker_rwsem-to-mutex.patch
revert-mm-vmscan-remove-shrinker_rwsem-from-synchronize_shrinkers.patch
revert-mm-vmscan-hold-write-lock-to-reparent-shrinker-nr_deferred.patch
revert-mm-shrinkers-make-count-and-scan-in-shrinker-debugfs-lockless.patch
revert-mm-vmscan-add-shrinker_srcu_generation.patch
revert-mm-vmscan-make-memcg-slab-shrink-lockless.patch
revert-mm-vmscan-make-global-slab-shrink-lockless.patch
mm-vmscan-move-shrinker_debugfs_remove-before-synchronize_srcu.patch
