+ revert-mm-vmscan-add-shrinker_srcu_generation.patch added to mm-hotfixes-unstable branch

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The patch titled
     Subject: Revert "mm: vmscan: add shrinker_srcu_generation"
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     revert-mm-vmscan-add-shrinker_srcu_generation.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-add-shrinker_srcu_generation.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: add shrinker_srcu_generation"
Date: Fri, 9 Jun 2023 08:15:16 +0000

This reverts commit 475733dda5aedba9e086379aafe6b5ffd53e8f5e.

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 revert the
shrinker_srcu related changes first.

[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/20230609081518.3039120-6-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 |   24 ++++--------------------
 1 file changed, 4 insertions(+), 20 deletions(-)

--- a/mm/vmscan.c~revert-mm-vmscan-add-shrinker_srcu_generation
+++ a/mm/vmscan.c
@@ -192,7 +192,6 @@ int vm_swappiness = 60;
 LIST_HEAD(shrinker_list);
 DECLARE_RWSEM(shrinker_rwsem);
 DEFINE_SRCU(shrinker_srcu);
-static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
 
 #ifdef CONFIG_MEMCG
 static int shrinker_nr_max;
@@ -818,7 +817,6 @@ void unregister_shrinker(struct shrinker
 	debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
 	up_write(&shrinker_rwsem);
 
-	atomic_inc(&shrinker_srcu_generation);
 	synchronize_srcu(&shrinker_srcu);
 
 	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
@@ -840,7 +838,6 @@ void synchronize_shrinkers(void)
 {
 	down_write(&shrinker_rwsem);
 	up_write(&shrinker_rwsem);
-	atomic_inc(&shrinker_srcu_generation);
 	synchronize_srcu(&shrinker_srcu);
 }
 EXPORT_SYMBOL(synchronize_shrinkers);
@@ -950,20 +947,18 @@ static unsigned long shrink_slab_memcg(g
 {
 	struct shrinker_info *info;
 	unsigned long ret, freed = 0;
-	int srcu_idx, generation;
-	int i = 0;
+	int srcu_idx;
+	int i;
 
 	if (!mem_cgroup_online(memcg))
 		return 0;
 
-again:
 	srcu_idx = srcu_read_lock(&shrinker_srcu);
 	info = shrinker_info_srcu(memcg, nid);
 	if (unlikely(!info))
 		goto unlock;
 
-	generation = atomic_read(&shrinker_srcu_generation);
-	for_each_set_bit_from(i, info->map, info->map_nr_max) {
+	for_each_set_bit(i, info->map, info->map_nr_max) {
 		struct shrink_control sc = {
 			.gfp_mask = gfp_mask,
 			.nid = nid,
@@ -1009,11 +1004,6 @@ again:
 				set_shrinker_bit(memcg, nid, i);
 		}
 		freed += ret;
-		if (atomic_read(&shrinker_srcu_generation) != generation) {
-			srcu_read_unlock(&shrinker_srcu, srcu_idx);
-			i++;
-			goto again;
-		}
 	}
 unlock:
 	srcu_read_unlock(&shrinker_srcu, srcu_idx);
@@ -1053,7 +1043,7 @@ static unsigned long shrink_slab(gfp_t g
 {
 	unsigned long ret, freed = 0;
 	struct shrinker *shrinker;
-	int srcu_idx, generation;
+	int srcu_idx;
 
 	/*
 	 * The root memcg might be allocated even though memcg is disabled
@@ -1067,7 +1057,6 @@ static unsigned long shrink_slab(gfp_t g
 
 	srcu_idx = srcu_read_lock(&shrinker_srcu);
 
-	generation = atomic_read(&shrinker_srcu_generation);
 	list_for_each_entry_srcu(shrinker, &shrinker_list, list,
 				 srcu_read_lock_held(&shrinker_srcu)) {
 		struct shrink_control sc = {
@@ -1080,11 +1069,6 @@ static unsigned long shrink_slab(gfp_t g
 		if (ret == SHRINK_EMPTY)
 			ret = 0;
 		freed += ret;
-
-		if (atomic_read(&shrinker_srcu_generation) != generation) {
-			freed = freed ? : 1;
-			break;
-		}
 	}
 
 	srcu_read_unlock(&shrinker_srcu, srcu_idx);
_

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




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