Commit 71c3ad65 authored by Qi Zheng's avatar Qi Zheng Committed by Andrew Morton
Browse files

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

This reverts commit f95bdb70.

Kernel test robot reports -88.8% regression in stress-ng.ramfs.ops_per_sec
test case [1], which is caused by commit f95bdb70 ("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: default avatarkernel test robot <yujie.liu@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@intel.com


Signed-off-by: default avatarQi 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 7cee3603

mm/vmscan.c

+16 −12
Original line number Diff line number Diff line
@@ -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;

}