| Message ID | 20210413065153.63431-6-songmuchun@bytedance.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | memcontrol code cleanup and simplification | expand |
On Mon, Apr 12, 2021 at 11:58 PM Muchun Song <songmuchun@bytedance.com> 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. > > 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> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Shakeel Butt <shakeelb@google.com>
On Tue, Apr 13, 2021 at 02:51:51PM +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. > > 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> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> It's a good one! It took me some time to realize that it's safe. Thanks! Acked-by: Roman Gushchin <guro@fb.com> > --- > mm/memcontrol.c | 20 ++++++-------------- > 1 file changed, 6 insertions(+), 14 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 1f807448233e..42d8c0f4ab1d 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 +327,12 @@ 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); > - > - /* 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); > - } > + /* 1) Ready to reparent active objcg. */ > + list_add(&objcg->list, &memcg->objcg_list); > + /* 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); > -- > 2.11.0 >
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 1f807448233e..42d8c0f4ab1d 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 +327,12 @@ 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); - - /* 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); - } + /* 1) Ready to reparent active objcg. */ + list_add(&objcg->list, &memcg->objcg_list); + /* 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);