| Message ID | 20211001190938.14050-4-longman@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm, memcg: Miscellaneous cleanups | expand |
On Fri, Oct 1, 2021 at 12:10 PM Waiman Long <longman@redhat.com> wrote: > > To ensure that a to-be-offlined memcg fetched from objcg remains > valid (has non-zero reference count) within a RCU critical section, > a synchronize_rcu() call is inserted at the end of memcg_offline_kmem(). > > With that change, we no longer need to use css_tryget() > in get_mem_cgroup_from_objcg() as the final css_put() in > css_killed_work_fn() would not have been called yet. > > The obj_cgroup_uncharge_pages() function is simplifed to perform > the whole uncharge operation within a RCU critical section saving a > css_get()/css_put() pair. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > mm/memcontrol.c | 16 +++++++++++----- > 1 file changed, 11 insertions(+), 5 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 8177f253a127..1dbb37d96e49 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2769,10 +2769,8 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg) > struct mem_cgroup *memcg; > > rcu_read_lock(); > -retry: > memcg = obj_cgroup_memcg(objcg); > - if (unlikely(!css_tryget(&memcg->css))) > - goto retry; > + css_get(&memcg->css); > rcu_read_unlock(); > > return memcg; > @@ -2947,13 +2945,14 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg, > { > struct mem_cgroup *memcg; > > - memcg = get_mem_cgroup_from_objcg(objcg); > + rcu_read_lock(); > + memcg = obj_cgroup_memcg(objcg); > > if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) > page_counter_uncharge(&memcg->kmem, nr_pages); > refill_stock(memcg, nr_pages); > > - css_put(&memcg->css); > + rcu_read_unlock(); > } > > /* > @@ -3672,6 +3671,13 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg) > memcg_drain_all_list_lrus(kmemcg_id, parent); > > memcg_free_cache_id(kmemcg_id); > + > + /* > + * To ensure that a to-be-offlined memcg fetched from objcg remains > + * valid within a RCU critical section, we need to wait here until > + * the a grace period has elapsed. > + */ > + synchronize_rcu(); This is called with cgroup_mutex held from css_offline path and synchronize_rcu() can be very expensive on a busy system, so, this will indirectly impact all the code paths which take cgroup_mutex. > } > #else > static int memcg_online_kmem(struct mem_cgroup *memcg) > -- > 2.18.1 >
On 10/1/21 4:24 PM, Shakeel Butt wrote: > On Fri, Oct 1, 2021 at 12:10 PM Waiman Long <longman@redhat.com> wrote: >> To ensure that a to-be-offlined memcg fetched from objcg remains >> valid (has non-zero reference count) within a RCU critical section, >> a synchronize_rcu() call is inserted at the end of memcg_offline_kmem(). >> >> With that change, we no longer need to use css_tryget() >> in get_mem_cgroup_from_objcg() as the final css_put() in >> css_killed_work_fn() would not have been called yet. >> >> The obj_cgroup_uncharge_pages() function is simplifed to perform >> the whole uncharge operation within a RCU critical section saving a >> css_get()/css_put() pair. >> >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> mm/memcontrol.c | 16 +++++++++++----- >> 1 file changed, 11 insertions(+), 5 deletions(-) >> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 8177f253a127..1dbb37d96e49 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -2769,10 +2769,8 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg) >> struct mem_cgroup *memcg; >> >> rcu_read_lock(); >> -retry: >> memcg = obj_cgroup_memcg(objcg); >> - if (unlikely(!css_tryget(&memcg->css))) >> - goto retry; >> + css_get(&memcg->css); >> rcu_read_unlock(); >> >> return memcg; >> @@ -2947,13 +2945,14 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg, >> { >> struct mem_cgroup *memcg; >> >> - memcg = get_mem_cgroup_from_objcg(objcg); >> + rcu_read_lock(); >> + memcg = obj_cgroup_memcg(objcg); >> >> if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) >> page_counter_uncharge(&memcg->kmem, nr_pages); >> refill_stock(memcg, nr_pages); >> >> - css_put(&memcg->css); >> + rcu_read_unlock(); >> } >> >> /* >> @@ -3672,6 +3671,13 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg) >> memcg_drain_all_list_lrus(kmemcg_id, parent); >> >> memcg_free_cache_id(kmemcg_id); >> + >> + /* >> + * To ensure that a to-be-offlined memcg fetched from objcg remains >> + * valid within a RCU critical section, we need to wait here until >> + * the a grace period has elapsed. >> + */ >> + synchronize_rcu(); > This is called with cgroup_mutex held from css_offline path and > synchronize_rcu() can be very expensive on a busy system, so, this > will indirectly impact all the code paths which take cgroup_mutex. > Yes, you are right. Just don't consider this patch for the time being. I will need to find a way to work around that. Thanks, Longman
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 8177f253a127..1dbb37d96e49 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2769,10 +2769,8 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg) struct mem_cgroup *memcg; rcu_read_lock(); -retry: memcg = obj_cgroup_memcg(objcg); - if (unlikely(!css_tryget(&memcg->css))) - goto retry; + css_get(&memcg->css); rcu_read_unlock(); return memcg; @@ -2947,13 +2945,14 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg, { struct mem_cgroup *memcg; - memcg = get_mem_cgroup_from_objcg(objcg); + rcu_read_lock(); + memcg = obj_cgroup_memcg(objcg); if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) page_counter_uncharge(&memcg->kmem, nr_pages); refill_stock(memcg, nr_pages); - css_put(&memcg->css); + rcu_read_unlock(); } /* @@ -3672,6 +3671,13 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg) memcg_drain_all_list_lrus(kmemcg_id, parent); memcg_free_cache_id(kmemcg_id); + + /* + * To ensure that a to-be-offlined memcg fetched from objcg remains + * valid within a RCU critical section, we need to wait here until + * the a grace period has elapsed. + */ + synchronize_rcu(); } #else static int memcg_online_kmem(struct mem_cgroup *memcg)
To ensure that a to-be-offlined memcg fetched from objcg remains valid (has non-zero reference count) within a RCU critical section, a synchronize_rcu() call is inserted at the end of memcg_offline_kmem(). With that change, we no longer need to use css_tryget() in get_mem_cgroup_from_objcg() as the final css_put() in css_killed_work_fn() would not have been called yet. The obj_cgroup_uncharge_pages() function is simplifed to perform the whole uncharge operation within a RCU critical section saving a css_get()/css_put() pair. Signed-off-by: Waiman Long <longman@redhat.com> --- mm/memcontrol.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-)