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

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@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>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a008d7f2d0fc8583aef32a5c6460b40c3c049ec6..5bf98d0a22c9a7107b76040b49fccd6aec353128 100644 (file)
--- a/mm/vmscan.c
+++ b/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 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;
 }