Hi Dave,
On 2023/8/8 10:44, Dave Chinner wrote:
On Mon, Aug 07, 2023 at 07:09:34PM +0800, Qi Zheng wrote:
Like global slab shrink, this commit also uses refcount+RCU method to make
memcg slab shrink lockless.
This patch does random code cleanups amongst the actual RCU changes.
Can you please move the cleanups to a spearate patch to reduce the
noise in this one?
Sure, will do.
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);
+}
This helper doesn't add value. It doesn't tell me that
rcu_read_lock() needs to be held when it is called, for one....
How about adding a comment or an assertion here?
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);
... whilst the original code here was obviously correct.
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)
Cleanups in a separate patch.
OK.
@@ -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;
}
This adds two rcu_read_lock() sections to every call to
do_shrink_slab(). It's not at all clear ifrom any of the other code
that do_shrink_slab() now has internal rcu_read_lock() sections....
The xchg_nr_deferred_memcg() will only be called in shrink_slab_memcg(),
so other code doesn't need to know that information?
@@ -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();
Why - what guarantees that the shrinker_info_unit exists at this
point? We hold no reference to it, we hold no reference to any
shrinker, etc. What provides this existence guarantee?
The shrinker_info_unit is never freed unless the memcg is destroyed.
Here we hold the refcount of this memcg (mem_cgroup_iter() -->
css_tryget()), so the shrinker_info_unit will not be freed.
+
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);
Ok, so why is this safe to call without holding the rcu read lock?
The global shrinker has to hold the rcu_read_lock() whilst calling
shrinker_put() to guarantee the validity of the list next pointer,
but we don't hold off RCU here so what guarantees a racing global
shrinker walk doesn't trip over this shrinker_put() call dropping
the refcount to zero and freeing occuring in a different context...
This will not be a problem, even if shrinker::refcount is reduced to
0 here, the racing global shrinker walk already holds the rcu lock.
shrink_slab shrink_slab_memcg
=========== =================
rcu_read_lock()
shrinker_put()
shrinker_put()
And in shrink_slab_memcg(), the shrinker is not required to traverse the
next bit in the shrinker_info_unit::map, so there is no need to hold the
rcu lock to ensure the existence of this 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;
With that, what makes the use of shrinker_info in
xchg_nr_deferred_memcg() in do_shrink_slab() coherent and valid?
Holding rcu lock can ensure that the old shrinker_info will not be
freed, and the shrinker_info_unit::nr_deferred can also be indexed from
the old shrinker_info::unit[x], so the updated nr_deferred will not be
lost.
Thanks,
Qi
-Dave.
