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Revert "mm: vmscan: make global slab shrink lockless"

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This reverts commit f95bdb700b.

Kernel test robot reports -88.8% regression in stress-ng.ramfs.ops_per_sec
test case [1], which is caused by commit f95bdb700b ("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@intel.com/
[2]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@dread.disaster.area/

Link: https://lkml.kernel.org/r/20230609081518.3039120-8-qi.zheng@linux.dev
Reported-by: kernel test robot <yujie.liu@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@intel.com
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Kirill Tkhai <tkhai@ya.ru>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Qi Zheng 2023-06-09 08:15:18 +00:00 committed by Andrew Morton
parent 7cee360319
commit 71c3ad65fa
1 changed files with 16 additions and 12 deletions
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28
mm/vmscan.c
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@ -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 shrinker *shrinker)
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 *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 gfp_mask, int nid,
{
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 gfp_mask, int nid,
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 gfp_mask, int nid,
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;
}