Revert "mm: vmscan: add shrinker_srcu_generation"

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@intel.com/
[2]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@dread.disaster.area/

Link: https://lkml.kernel.org/r/20230609081518.3039120-6-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>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d1d309f..50775b7 100644 (file)
--- 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 @@ again:
                                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);