[PATCH v4 46/48] mm: shrinker: make memcg slab shrink lockless

From: Qi Zheng
Date: Mon Aug 07 2023 - 07:38:41 EST

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 | 80 ++++++++++++++++++++++++++++++++++-----------------
1 file changed, 54 insertions(+), 26 deletions(-)

diff --git a/mm/shrinker.c b/mm/shrinker.c
index d318f5621862..fee6f62904fb 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -107,6 +107,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
lockdep_is_held(&shrinker_rwsem));
}

+static struct shrinker_info *shrinker_info_rcu(struct mem_cgroup *memcg,
+ int nid)
+{
+ return rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
+}
+
static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_size,
int old_size, int new_nr_max)
{
@@ -198,7 +204,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
struct shrinker_info_unit *unit;

rcu_read_lock();
- info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
+ info = shrinker_info_rcu(memcg, nid);
unit = info->unit[shriner_id_to_index(shrinker_id)];
if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
/* Pairs with smp mb in shrink_slab() */
@@ -211,7 +217,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)

static DEFINE_IDR(shrinker_idr);

-static int prealloc_memcg_shrinker(struct shrinker *shrinker)
+static int shrinker_memcg_alloc(struct shrinker *shrinker)
{
int id, ret = -ENOMEM;

@@ -219,7 +225,6 @@ static int prealloc_memcg_shrinker(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;
@@ -237,7 +242,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
return ret;
}

-static void unregister_memcg_shrinker(struct shrinker *shrinker)
+static void shrinker_memcg_remove(struct shrinker *shrinker)
{
int id = shrinker->id;

@@ -253,10 +258,15 @@ 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 = shrinker_info_rcu(memcg, nid);
unit = info->unit[shriner_id_to_index(shrinker->id)];
- return atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0);
+ nr_deferred = atomic_long_xchg(&unit->nr_deferred[shriner_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,
@@ -264,10 +274,16 @@ 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 = shrinker_info_rcu(memcg, nid);
unit = info->unit[shriner_id_to_index(shrinker->id)];
- return atomic_long_add_return(nr, &unit->nr_deferred[shriner_id_to_offset(shrinker->id)]);
+ nr_deferred =
+ atomic_long_add_return(nr, &unit->nr_deferred[shriner_id_to_offset(shrinker->id)]);
+ rcu_read_unlock();
+
+ return nr_deferred;
}

void reparent_shrinker_deferred(struct mem_cgroup *memcg)
@@ -299,12 +315,12 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
up_read(&shrinker_rwsem);
}
#else
-static int prealloc_memcg_shrinker(struct shrinker *shrinker)
+static int shrinker_memcg_alloc(struct shrinker *shrinker)
{
return -ENOSYS;
}

-static void unregister_memcg_shrinker(struct shrinker *shrinker)
+static void shrinker_memcg_remove(struct shrinker *shrinker)
{
}

@@ -464,18 +480,23 @@ 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);
+again:
+ rcu_read_lock();
+ info = shrinker_info_rcu(memcg, nid);
if (unlikely(!info))
goto unlock;

- for (; index < shriner_id_to_index(info->map_nr_max); index++) {
+ if (index < shriner_id_to_index(info->map_nr_max)) {
struct shrinker_info_unit *unit;

unit = info->unit[index];

+ /*
+ * The shrinker_info_unit will not be freed, so we can
+ * safely release the RCU lock here.
+ */
+ rcu_read_unlock();
+
for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
@@ -485,12 +506,14 @@ 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() &&
@@ -523,15 +546,20 @@ 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);
}
+
+ /*
+ * 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 reacquire the
+ * shrinker_info.
+ */
+ index++;
+ goto again;
}
unlock:
- up_read(&shrinker_rwsem);
+ rcu_read_unlock();
return freed;
}
#else /* !CONFIG_MEMCG */
@@ -638,7 +666,7 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...)
shrinker->flags = flags | SHRINKER_ALLOCATED;

if (flags & SHRINKER_MEMCG_AWARE) {
- err = prealloc_memcg_shrinker(shrinker);
+ err = shrinker_memcg_alloc(shrinker);
if (err == -ENOSYS)
shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
else if (err == 0)
@@ -731,7 +759,7 @@ void shrinker_free(struct shrinker *shrinker)
}

if (shrinker->flags & SHRINKER_MEMCG_AWARE)
- unregister_memcg_shrinker(shrinker);
+ shrinker_memcg_remove(shrinker);
up_write(&shrinker_rwsem);

if (debugfs_entry)
--
2.30.2