The patch titled
Subject: Revert "mm: shrinkers: make count and scan in shrinker debugfs lockless"
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
revert-mm-shrinkers-make-count-and-scan-in-shrinker-debugfs-lockless.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/revert-mm-shrinkers-make-count-and-scan-in-shrinker-debugfs-lockless.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: shrinkers: make count and scan in shrinker debugfs lockless"
Date: Fri, 9 Jun 2023 08:15:15 +0000
This reverts commit 20cd1892fcc3efc10a7ac327cc3790494bec46b5.
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/20230609081518.3039120-5-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/shrinker_debug.c | 25 +++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)
--- a/mm/shrinker_debug.c~revert-mm-shrinkers-make-count-and-scan-in-shrinker-debugfs-lockless
+++ a/mm/shrinker_debug.c
@@ -5,12 +5,10 @@
#include <linux/seq_file.h>
#include <linux/shrinker.h>
#include <linux/memcontrol.h>
-#include <linux/srcu.h>
/* defined in vmscan.c */
extern struct rw_semaphore shrinker_rwsem;
extern struct list_head shrinker_list;
-extern struct srcu_struct shrinker_srcu;
static DEFINE_IDA(shrinker_debugfs_ida);
static struct dentry *shrinker_debugfs_root;
@@ -51,13 +49,18 @@ static int shrinker_debugfs_count_show(s
struct mem_cgroup *memcg;
unsigned long total;
bool memcg_aware;
- int ret = 0, nid, srcu_idx;
+ int ret, nid;
count_per_node = kcalloc(nr_node_ids, sizeof(unsigned long), GFP_KERNEL);
if (!count_per_node)
return -ENOMEM;
- srcu_idx = srcu_read_lock(&shrinker_srcu);
+ ret = down_read_killable(&shrinker_rwsem);
+ if (ret) {
+ kfree(count_per_node);
+ return ret;
+ }
+ rcu_read_lock();
memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
@@ -88,7 +91,8 @@ static int shrinker_debugfs_count_show(s
}
} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
- srcu_read_unlock(&shrinker_srcu, srcu_idx);
+ rcu_read_unlock();
+ up_read(&shrinker_rwsem);
kfree(count_per_node);
return ret;
@@ -111,8 +115,9 @@ static ssize_t shrinker_debugfs_scan_wri
.gfp_mask = GFP_KERNEL,
};
struct mem_cgroup *memcg = NULL;
- int nid, srcu_idx;
+ int nid;
char kbuf[72];
+ ssize_t ret;
read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
if (copy_from_user(kbuf, buf, read_len))
@@ -141,7 +146,11 @@ static ssize_t shrinker_debugfs_scan_wri
return -EINVAL;
}
- srcu_idx = srcu_read_lock(&shrinker_srcu);
+ ret = down_read_killable(&shrinker_rwsem);
+ if (ret) {
+ mem_cgroup_put(memcg);
+ return ret;
+ }
sc.nid = nid;
sc.memcg = memcg;
@@ -150,7 +159,7 @@ static ssize_t shrinker_debugfs_scan_wri
shrinker->scan_objects(shrinker, &sc);
- srcu_read_unlock(&shrinker_srcu, srcu_idx);
+ up_read(&shrinker_rwsem);
mem_cgroup_put(memcg);
return size;
_
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
