diff mbox series

[RFC,v2,05/18] mm: memcontrol: simplify the logic of objcg pinning memcg

Message ID 20210409122959.82264-6-songmuchun@bytedance.com (mailing list archive)
State New, archived
Headers show
Series Use obj_cgroup APIs to charge the LRU pages | expand

Commit Message

Muchun Song April 9, 2021, 12:29 p.m. UTC 
The obj_cgroup_release() and memcg_reparent_objcgs() are serialized by
the css_set_lock. We do not need to care about objcg->memcg being
released in the process of obj_cgroup_release(). So there is no need
to pin memcg before releasing objcg. Remove those pinning logic to
simplfy the code.

There are only two places that modifies the objcg->memcg. One is the
initialization to objcg->memcg in the memcg_online_kmem(), another
is objcgs reparenting in the memcg_reparent_objcgs(). It is also
impossible for the two to run in parallel. So xchg() is unnecessary
and it is enough to use WRITE_ONCE().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memcontrol.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

Comments

Johannes Weiner April 9, 2021, 4:27 p.m. UTC | #1 
On Fri, Apr 09, 2021 at 08:29:46PM +0800, Muchun Song wrote:
> The obj_cgroup_release() and memcg_reparent_objcgs() are serialized by
> the css_set_lock. We do not need to care about objcg->memcg being
> released in the process of obj_cgroup_release(). So there is no need
> to pin memcg before releasing objcg. Remove those pinning logic to
> simplfy the code.

Hm yeah, it's not clear to me why inherited objcgs pinned the memcg in
the first place, since they are reparented during memcg deletion and
therefor have no actual impact on the memcg's lifetime.

> There are only two places that modifies the objcg->memcg. One is the
> initialization to objcg->memcg in the memcg_online_kmem(), another
> is objcgs reparenting in the memcg_reparent_objcgs(). It is also
> impossible for the two to run in parallel. So xchg() is unnecessary
> and it is enough to use WRITE_ONCE().

Good catch.

> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Looks like a nice cleanup / simplification:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
diff mbox series

Patch

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1f807448233e..90c1ac58c64c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -261,7 +261,6 @@  static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 static void obj_cgroup_release(struct percpu_ref *ref)
 {
 	struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
-	struct mem_cgroup *memcg;
 	unsigned int nr_bytes;
 	unsigned int nr_pages;
 	unsigned long flags;
@@ -291,11 +290,9 @@  static void obj_cgroup_release(struct percpu_ref *ref)
 	nr_pages = nr_bytes >> PAGE_SHIFT;
 
 	spin_lock_irqsave(&css_set_lock, flags);
-	memcg = obj_cgroup_memcg(objcg);
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
 	list_del(&objcg->list);
-	mem_cgroup_put(memcg);
 	spin_unlock_irqrestore(&css_set_lock, flags);
 
 	percpu_ref_exit(ref);
@@ -330,17 +327,14 @@  static void memcg_reparent_objcgs(struct mem_cgroup *memcg,
 
 	spin_lock_irq(&css_set_lock);
 
-	/* Move active objcg to the parent's list */
-	xchg(&objcg->memcg, parent);
-	css_get(&parent->css);
-	list_add(&objcg->list, &parent->objcg_list);
+	/* 1) Ready to reparent active objcg. */
+	list_add(&objcg->list, &memcg->objcg_list);
 
-	/* Move already reparented objcgs to the parent's list */
-	list_for_each_entry(iter, &memcg->objcg_list, list) {
-		css_get(&parent->css);
-		xchg(&iter->memcg, parent);
-		css_put(&memcg->css);
-	}
+	/* 2) Reparent active objcg and already reparented objcgs to parent. */
+	list_for_each_entry(iter, &memcg->objcg_list, list)
+		WRITE_ONCE(iter->memcg, parent);
+
+	/* 3) Move already reparented objcgs to the parent's list */
 	list_splice(&memcg->objcg_list, &parent->objcg_list);
 
 	spin_unlock_irq(&css_set_lock);