From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
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/
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>
---
mm/vmscan.c | 24 ++++--------------------
1 file changed, 4 insertions(+), 20 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d1d309fc3212..50775b73d0c7 100644
--- a/mm/vmscan.c
+++ b/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 *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(gfp_t gfp_mask, int nid,
{
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 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
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 gfp_mask, int nid,
{
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 gfp_mask, int nid,
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 gfp_mask, int nid,
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);
--
2.30.2
