[PATCH v6 43/45] mm: shrinker: make memcg slab shrink lockless

Qi Zheng
2023-09-11T21:40:04Z

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:

33.15% [kernel] [k] down_read_trylock
25.38% [kernel] [k] shrink_slab
21.75% [kernel] [k] up_read
4.45% [kernel] [k] _find_next_bit
2.27% [kernel] [k] do_shrink_slab
1.80% [kernel] [k] intel_idle_irq
1.79% [kernel] [k] shrink_lruvec
0.67% [kernel] [k] xas_descend
0.41% [kernel] [k] mem_cgroup_iter
0.40% [kernel] [k] shrink_node
0.38% [kernel] [k] list_lru_count_one

2) After applying this patchset:

64.56% [kernel] [k] shrink_slab
12.18% [kernel] [k] do_shrink_slab
3.30% [kernel] [k] __rcu_read_unlock
2.61% [kernel] [k] shrink_lruvec
2.49% [kernel] [k] __rcu_read_lock
1.93% [kernel] [k] intel_idle_irq
0.89% [kernel] [k] shrink_node
0.81% [kernel] [k] mem_cgroup_iter
0.77% [kernel] [k] mem_cgroup_calculate_protection
0.66% [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@bytedance.com>
---
mm/shrinker.c | 85 +++++++++++++++++++++++++++++++++++++++------------
1 file changed, 66 insertions(+), 19 deletions(-)

diff --git a/mm/shrinker.c b/mm/shrinker.c
index 82dc61133c5b..ad64cac5248c 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