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 | 121 +++++++++++++++++++++++++++-----------------
mm/shrinker_debug.c | 4 +-
2 files changed, 76 insertions(+), 49 deletions(-)
diff --git a/mm/shrinker.c b/mm/shrinker.c
index 8e3334749552..744361afd520 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)
{
@@ -152,11 +158,11 @@ static int expand_shrinker_info(int new_id)
int new_size, old_size = 0;
struct mem_cgroup *memcg;
+ down_write(&shrinker_rwsem);
+
if (!root_mem_cgroup)
goto out;
- lockdep_assert_held(&shrinker_rwsem);
-
new_size = shrinker_unit_size(new_nr_max);
old_size = shrinker_unit_size(shrinker_nr_max);
@@ -173,6 +179,8 @@ static int expand_shrinker_info(int new_id)
if (!ret)
shrinker_nr_max = new_nr_max;
+ up_write(&shrinker_rwsem);
+
return ret;
}
@@ -198,7 +206,7 @@ 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() */
@@ -211,39 +219,40 @@ 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;
+ int id;
if (mem_cgroup_disabled())
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);
+ spin_lock(&shrinker_lock);
+ id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_ATOMIC);
+ spin_unlock(&shrinker_lock);
+
if (id < 0)
- goto unlock;
+ return -ENOMEM;
if (id >= shrinker_nr_max) {
if (expand_shrinker_info(id)) {
+ spin_lock(&shrinker_lock);
idr_remove(&shrinker_idr, id);
- goto unlock;
+ spin_unlock(&shrinker_lock);
+ return -ENOMEM;
}
}
shrinker->id = id;
- ret = 0;
-unlock:
- up_write(&shrinker_rwsem);
- return ret;
+
+ return 0;
}
-static void unregister_memcg_shrinker(struct shrinker *shrinker)
+static void shrinker_memcg_remove(struct shrinker *shrinker)
{
int id = shrinker->id;
BUG_ON(id < 0);
- lockdep_assert_held(&shrinker_rwsem);
+ lockdep_assert_held(&shrinker_lock);
idr_remove(&shrinker_idr, id);
}
@@ -253,10 +262,15 @@ 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 +278,16 @@ 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 +319,12 @@ 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)
{
}
@@ -458,6 +478,8 @@ void shrinker_put(struct shrinker *shrinker)
if (refcount_dec_and_test(&shrinker->refcount)) {
spin_lock(&shrinker_lock);
list_del_rcu(&shrinker->list);
+ if (shrinker->flags & SHRINKER_MEMCG_AWARE)
+ shrinker_memcg_remove(shrinker);
spin_unlock(&shrinker_lock);
kfree(shrinker->nr_deferred);
@@ -476,18 +498,23 @@ 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,
@@ -497,13 +524,14 @@ 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 ||
- !READ_ONCE(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() &&
@@ -536,15 +564,20 @@ 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 */
@@ -652,7 +685,7 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...)
shrinker->flags = flags;
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)
@@ -697,9 +730,9 @@ void shrinker_free_non_registered(struct shrinker *shrinker)
shrinker->name = NULL;
#endif
if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
- down_write(&shrinker_rwsem);
- unregister_memcg_shrinker(shrinker);
- up_write(&shrinker_rwsem);
+ spin_lock(&shrinker_lock);
+ shrinker_memcg_remove(shrinker);
+ spin_unlock(&shrinker_lock);
}
kfree(shrinker->nr_deferred);
@@ -731,13 +764,7 @@ void shrinker_unregister(struct shrinker *shrinker)
return;
WRITE_ONCE(shrinker->registered, false);
-
- down_write(&shrinker_rwsem);
- if (shrinker->flags & SHRINKER_MEMCG_AWARE)
- unregister_memcg_shrinker(shrinker);
debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
- up_write(&shrinker_rwsem);
-
shrinker_debugfs_remove(debugfs_entry, debugfs_id);
shrinker_put(shrinker);
diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index c5573066adbf..badda35464c3 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -246,8 +246,7 @@ struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
{
struct dentry *entry = shrinker->debugfs_entry;
- lockdep_assert_held(&shrinker_rwsem);
-
+ down_write(&shrinker_rwsem);
kfree_const(shrinker->name);
shrinker->name = NULL;
@@ -258,6 +257,7 @@ struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
*/
smp_wmb();
shrinker->debugfs_entry = NULL;
+ up_write(&shrinker_rwsem);
return entry;
}
--
2.30.2
