From: Waiman Long <llong@redhat.com>
To: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@kernel.org>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>, Roman Gushchin <guro@fb.com>,
LKML <linux-kernel@vger.kernel.org>,
Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH 3/3] mm, memcg: Ensure valid memcg from objcg within a RCU critical section
Date: Fri, 1 Oct 2021 16:34:09 -0400 [thread overview]
Message-ID: <1d1dd020-e86e-8478-5fea-0e4a97100667@redhat.com> (raw)
In-Reply-To: <CALvZod42VDSMy4E47snF-8yToSkt7no1h=KnYmQnH2dz2CDPLQ@mail.gmail.com>
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
prev parent reply other threads:[~2021-10-01 20:34 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-01 19:09 [PATCH 0/3] mm, memcg: Miscellaneous cleanups Waiman Long
2021-10-01 19:09 ` [PATCH 1/3] mm, memcg: Don't put offlined memcg into local stock Waiman Long
2021-10-01 21:17 ` kernel test robot
2021-10-01 23:06 ` kernel test robot
2021-10-01 23:51 ` Roman Gushchin
2021-10-02 1:54 ` Waiman Long
2022-01-31 3:55 ` Waiman Long
2022-01-31 17:01 ` Roman Gushchin
2022-01-31 17:09 ` Waiman Long
2022-01-31 17:15 ` Waiman Long
2022-01-31 17:19 ` Roman Gushchin
2022-01-31 17:25 ` Waiman Long
2022-01-31 18:00 ` Shakeel Butt
2022-01-31 17:15 ` Roman Gushchin
2021-10-01 19:09 ` [PATCH 2/3] mm, memcg: Remove obsolete memcg_free_kmem() Waiman Long
2021-10-02 0:01 ` Roman Gushchin
2021-10-02 2:03 ` Waiman Long
2021-10-01 19:09 ` [PATCH 3/3] mm, memcg: Ensure valid memcg from objcg within a RCU critical section Waiman Long
2021-10-01 20:24 ` Shakeel Butt
2021-10-01 20:34 ` Waiman Long [this message]
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