linux-kernel - [PATCH 3/7] Revert "mm: vmscan: hold write lock to reparent shrinker nr

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Message-Id: <20230609081518.3039120-4-qi.zheng@linux.dev>
Date:   Fri,  9 Jun 2023 08:15:14 +0000
From:   Qi Zheng <qi.zheng@...ux.dev>
To:     akpm@...ux-foundation.org
Cc:     david@...morbit.com, tkhai@...ru, roman.gushchin@...ux.dev,
        vbabka@...e.cz, muchun.song@...ux.dev, yujie.liu@...el.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH 3/7] Revert "mm: vmscan: hold write lock to reparent shrinker nr_deferred"

From: Qi Zheng <zhengqi.arch@...edance.com>

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/

Reported-by: kernel test robot <yujie.liu@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@intel.com
Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0ba0e1180f3f..d1d309fc3212 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)
-- 
2.30.2