[PATCH v5 43/45] mm: shrinker: make memcg slab shrink lockless

[Qi Zheng]

Like global slab shrink, this commit also uses refcount+RCU method to make
memcg slab shrink lockless.

Use the following script to do slab shrink stress test:

```

DIR="/root/shrinker/memcg/mnt"

do_create()
{
    mkdir -p /sys/fs/cgroup/memory/test
    echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
    for i in `seq 0 $1`;
    do
        mkdir -p /sys/fs/cgroup/memory/test/$i;
        echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
        mkdir -p $DIR/$i;
    done
}

do_mount()
{
    for i in `seq $1 $2`;
    do
        mount -t tmpfs $i $DIR/$i;
    done
}

do_touch()
{
    for i in `seq $1 $2`;
    do
        echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
        dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 &
    done
}

case "$1" in
  touch)
    do_touch $2 $3
    ;;
  test)
    do_create 4000
    do_mount 0 4000
    do_touch 0 3000
    ;;
  *)
    exit 1
    ;;
esac
```

Save the above script, then run test and touch commands. Then we can use
the following perf command to view hotspots:

perf top -U -F 999

1) Before applying this patchset:

  40.44%  [kernel]            [k] down_read_trylock
  17.59%  [kernel]            [k] up_read
  13.64%  [kernel]            [k] pv_native_safe_halt
  11.90%  [kernel]            [k] shrink_slab
   8.21%  [kernel]            [k] idr_find
   2.71%  [kernel]            [k] _find_next_bit
   1.36%  [kernel]            [k] shrink_node
   0.81%  [kernel]            [k] shrink_lruvec
   0.80%  [kernel]            [k] __radix_tree_lookup
   0.50%  [kernel]            [k] do_shrink_slab
   0.21%  [kernel]            [k] list_lru_count_one
   0.16%  [kernel]            [k] mem_cgroup_iter

2) After applying this patchset:

  60.17%  [kernel]           [k] shrink_slab
  20.42%  [kernel]           [k] pv_native_safe_halt
   3.03%  [kernel]           [k] do_shrink_slab
   2.73%  [kernel]           [k] shrink_node
   2.27%  [kernel]           [k] shrink_lruvec
   2.00%  [kernel]           [k] __rcu_read_unlock
   1.92%  [kernel]           [k] mem_cgroup_iter
   0.98%  [kernel]           [k] __rcu_read_lock
   0.91%  [kernel]           [k] osq_lock
   0.63%  [kernel]           [k] mem_cgroup_calculate_protection
   0.55%  [kernel]           [k] shrinker_put
   0.46%  [kernel]           [k] list_lru_count_one

We can see that the first perf hotspot becomes shrink_slab, which is what
we expect.

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 mm/shrinker.c | 85 +++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 66 insertions(+), 19 deletions(-)

diff --git a/mm/shrinker.c b/mm/shrinker.c
index 2b8c1f1bbf2d..a66e2a30cc16 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -218,7 +218,6 @@ static int shrinker_memcg_alloc(struct shrinker *shrinker)
 		return -ENOSYS;
 
 	down_write(&shrinker_rwsem);
-	/* This may call shrinker, so it must use down_read_trylock() */
 	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
 	if (id < 0)
 		goto unlock;
@@ -252,10 +251,15 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker,
 {
 	struct shrinker_info *info;
 	struct shrinker_info_unit *unit;
+	long nr_deferred;
 
-	info = shrinker_info_protected(memcg, nid);
+	rcu_read_lock();
+	info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
 	unit = info->unit[shrinker_id_to_index(shrinker->id)];
-	return atomic_long_xchg(&unit->nr_deferred[shrinker_id_to_offset(shrinker->id)], 0);
+	nr_deferred = atomic_long_xchg(&unit->nr_deferred[shrinker_id_to_offset(shrinker->id)], 0);
+	rcu_read_unlock();
+
+	return nr_deferred;
 }
 
 static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
@@ -263,10 +267,16 @@ static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
 {
 	struct shrinker_info *info;
 	struct shrinker_info_unit *unit;
+	long nr_deferred;
 
-	info = shrinker_info_protected(memcg, nid);
+	rcu_read_lock();
+	info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
 	unit = info->unit[shrinker_id_to_index(shrinker->id)];
-	return atomic_long_add_return(nr, &unit->nr_deferred[shrinker_id_to_offset(shrinker->id)]);
+	nr_deferred =
+		atomic_long_add_return(nr, &unit->nr_deferred[shrinker_id_to_offset(shrinker->id)]);
+	rcu_read_unlock();
+
+	return nr_deferred;
 }
 
 void reparent_shrinker_deferred(struct mem_cgroup *memcg)
@@ -463,18 +473,54 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 	if (!mem_cgroup_online(memcg))
 		return 0;
 
-	if (!down_read_trylock(&shrinker_rwsem))
-		return 0;
-
-	info = shrinker_info_protected(memcg, nid);
+	/*
+	 * lockless algorithm of memcg shrink.
+	 *
+	 * The shrinker_info may be freed asynchronously via RCU in the
+	 * expand_one_shrinker_info(), so the rcu_read_lock() needs to be used
+	 * to ensure the existence of the shrinker_info.
+	 *
+	 * The shrinker_info_unit is never freed unless its corresponding memcg
+	 * is destroyed. Here we already hold the refcount of memcg, so the
+	 * memcg will not be destroyed, and of course shrinker_info_unit will
+	 * not be freed.
+	 *
+	 * So in the memcg shrink:
+	 *  step 1: use rcu_read_lock() to guarantee existence of the
+	 *          shrinker_info.
+	 *  step 2: after getting shrinker_info_unit we can safely release the
+	 *          RCU lock.
+	 *  step 3: traverse the bitmap and calculate shrinker_id
+	 *  step 4: use rcu_read_lock() to guarantee existence of the shrinker.
+	 *  step 5: use shrinker_id to find the shrinker, then use
+	 *          shrinker_try_get() to guarantee existence of the shrinker,
+	 *          then we can release the RCU lock to do do_shrink_slab() that
+	 *          may sleep.
+	 *  step 6: do shrinker_put() paired with step 5 to put the refcount,
+	 *          if the refcount reaches 0, then wake up the waiter in
+	 *          shrinker_free() by calling complete().
+	 *          Note: here is different from the global shrink, we don't
+	 *                need to acquire the RCU lock to guarantee existence of
+	 *                the shrinker, because we don't need to use this
+	 *                shrinker to traverse the next shrinker in the bitmap.
+	 *  step 7: we have already exited the read-side of rcu critical section
+	 *          before calling do_shrink_slab(), the shrinker_info may be
+	 *          released in expand_one_shrinker_info(), so go back to step 1
+	 *          to reacquire the shrinker_info.
+	 */
+again:
+	rcu_read_lock();
+	info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
 	if (unlikely(!info))
 		goto unlock;
 
-	for (; index < shrinker_id_to_index(info->map_nr_max); index++) {
+	if (index < shrinker_id_to_index(info->map_nr_max)) {
 		struct shrinker_info_unit *unit;
 
 		unit = info->unit[index];
 
+		rcu_read_unlock();
+
 		for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) {
 			struct shrink_control sc = {
 				.gfp_mask = gfp_mask,
@@ -484,12 +530,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			struct shrinker *shrinker;
 			int shrinker_id = calc_shrinker_id(index, offset);
 
+			rcu_read_lock();
 			shrinker = idr_find(&shrinker_idr, shrinker_id);
-			if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) {
-				if (!shrinker)
-					clear_bit(offset, unit->map);
+			if (unlikely(!shrinker || !shrinker_try_get(shrinker))) {
+				clear_bit(offset, unit->map);
+				rcu_read_unlock();
 				continue;
 			}
+			rcu_read_unlock();
 
 			/* Call non-slab shrinkers even though kmem is disabled */
 			if (!memcg_kmem_online() &&
@@ -522,15 +570,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 					set_shrinker_bit(memcg, nid, shrinker_id);
 			}
 			freed += ret;
-
-			if (rwsem_is_contended(&shrinker_rwsem)) {
-				freed = freed ? : 1;
-				goto unlock;
-			}
+			shrinker_put(shrinker);
 		}
+
+		index++;
+		goto again;
 	}
 unlock:
-	up_read(&shrinker_rwsem);
+	rcu_read_unlock();
 	return freed;
 }
 #else /* !CONFIG_MEMCG */
-- 
2.30.2