The patch titled
Subject: Revert "mm: vmscan: make global slab shrink lockless"
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
revert-mm-vmscan-make-global-slab-shrink-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-vmscan-make-global-slab-shrink-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: vmscan: make global slab shrink lockless"
Date: Fri, 9 Jun 2023 08:15:18 +0000
This reverts commit f95bdb700bc6bb74e1199b1f5f90c613e152cfa7.
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.
After discussion, 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/20230609081518.3039120-8-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 | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)
--- a/mm/vmscan.c~revert-mm-vmscan-make-global-slab-shrink-lockless
+++ a/mm/vmscan.c
@@ -57,7 +57,6 @@
#include <linux/khugepaged.h>
#include <linux/rculist_nulls.h>
#include <linux/random.h>
-#include <linux/srcu.h>
#include <asm/tlbflush.h>
#include <asm/div64.h>
@@ -191,7 +190,6 @@ int vm_swappiness = 60;
LIST_HEAD(shrinker_list);
DECLARE_RWSEM(shrinker_rwsem);
-DEFINE_SRCU(shrinker_srcu);
#ifdef CONFIG_MEMCG
static int shrinker_nr_max;
@@ -742,7 +740,7 @@ void free_prealloced_shrinker(struct shr
void register_shrinker_prepared(struct shrinker *shrinker)
{
down_write(&shrinker_rwsem);
- list_add_tail_rcu(&shrinker->list, &shrinker_list);
+ list_add_tail(&shrinker->list, &shrinker_list);
shrinker->flags |= SHRINKER_REGISTERED;
shrinker_debugfs_add(shrinker);
up_write(&shrinker_rwsem);
@@ -797,15 +795,13 @@ void unregister_shrinker(struct shrinker
return;
down_write(&shrinker_rwsem);
- list_del_rcu(&shrinker->list);
+ list_del(&shrinker->list);
shrinker->flags &= ~SHRINKER_REGISTERED;
if (shrinker->flags & SHRINKER_MEMCG_AWARE)
unregister_memcg_shrinker(shrinker);
debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
up_write(&shrinker_rwsem);
- synchronize_srcu(&shrinker_srcu);
-
shrinker_debugfs_remove(debugfs_entry, debugfs_id);
kfree(shrinker->nr_deferred);
@@ -825,7 +821,6 @@ void synchronize_shrinkers(void)
{
down_write(&shrinker_rwsem);
up_write(&shrinker_rwsem);
- synchronize_srcu(&shrinker_srcu);
}
EXPORT_SYMBOL(synchronize_shrinkers);
@@ -1036,7 +1031,6 @@ static unsigned long shrink_slab(gfp_t g
{
unsigned long ret, freed = 0;
struct shrinker *shrinker;
- int srcu_idx;
/*
* The root memcg might be allocated even though memcg is disabled
@@ -1048,10 +1042,10 @@ static unsigned long shrink_slab(gfp_t g
if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
- srcu_idx = srcu_read_lock(&shrinker_srcu);
+ if (!down_read_trylock(&shrinker_rwsem))
+ goto out;
- list_for_each_entry_srcu(shrinker, &shrinker_list, list,
- srcu_read_lock_held(&shrinker_srcu)) {
+ list_for_each_entry(shrinker, &shrinker_list, list) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -1062,9 +1056,19 @@ static unsigned long shrink_slab(gfp_t g
if (ret == SHRINK_EMPTY)
ret = 0;
freed += ret;
+ /*
+ * Bail out if someone want to register a new shrinker to
+ * prevent the registration from being stalled for long periods
+ * by parallel ongoing shrinking.
+ */
+ if (rwsem_is_contended(&shrinker_rwsem)) {
+ freed = freed ? : 1;
+ break;
+ }
}
- srcu_read_unlock(&shrinker_srcu, srcu_idx);
+ up_read(&shrinker_rwsem);
+out:
cond_resched();
return freed;
}
_
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
