Re: [External] [PATCH v4 5/5] mm/memcg: Improve refill_obj_stock() performance

[Waiman Long <llong@redhat.com>]

On 4/19/21 11:00 AM, Shakeel Butt wrote:
> On Sun, Apr 18, 2021 at 11:07 PM Muchun Song <songmuchun@bytedance.com> wrote:
>> On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@redhat.com> wrote:
>>> There are two issues with the current refill_obj_stock() code. First of
>>> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
>>> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
>>> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
>>> be used again which leads to another call to drain_obj_stock() and
>>> obj_cgroup_get() as well as atomically retrieve the available byte from
>>> obj_cgroup. That is costly. Instead, we should just uncharge the excess
>>> pages, reduce the stock bytes and be done with it. The drain_obj_stock()
>>> function should only be called when obj_cgroup changes.
>>>
>>> Secondly, when charging an object of size not less than a page in
>>> obj_cgroup_charge(), it is possible that the remaining bytes to be
>>> refilled to the stock will overflow a page and cause refill_obj_stock()
>>> to uncharge 1 page. To avoid the additional uncharge in this case,
>>> a new overfill flag is added to refill_obj_stock() which will be set
>>> when called from obj_cgroup_charge().
>>>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> ---
>>>   mm/memcontrol.c | 23 +++++++++++++++++------
>>>   1 file changed, 17 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index a6dd18f6d8a8..d13961352eef 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>>>          return false;
>>>   }
>>>
>>> -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>>> +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
>>> +                            bool overfill)
>>>   {
>>>          unsigned long flags;
>>>          struct obj_stock *stock = get_obj_stock(&flags);
>>> +       unsigned int nr_pages = 0;
>>>
>>>          if (stock->cached_objcg != objcg) { /* reset if necessary */
>>> -               drain_obj_stock(stock);
>>> +               if (stock->cached_objcg)
>>> +                       drain_obj_stock(stock);
>>>                  obj_cgroup_get(objcg);
>>>                  stock->cached_objcg = objcg;
>>>                  stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
>>>          }
>>>          stock->nr_bytes += nr_bytes;
>>>
>>> -       if (stock->nr_bytes > PAGE_SIZE)
>>> -               drain_obj_stock(stock);
>>> +       if (!overfill && (stock->nr_bytes > PAGE_SIZE)) {
>>> +               nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>>> +               stock->nr_bytes &= (PAGE_SIZE - 1);
>>> +       }
>>>
>>>          put_obj_stock(flags);
>>> +
>>> +       if (nr_pages) {
>>> +               rcu_read_lock();
>>> +               __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages);
>>> +               rcu_read_unlock();
>>> +       }
>> It is not safe to call __memcg_kmem_uncharge() under rcu lock
>> and without holding a reference to memcg. More details can refer
>> to the following link.
>>
>> https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@bytedance.com/
>>
>> In the above patchset, we introduce obj_cgroup_uncharge_pages to
>> uncharge some pages from object cgroup. You can use this safe
>> API.
>>
> I would recommend just rebase the patch series over the latest mm tree.
>
I see, I will rebase it to the latest mm tree.

Thanks,
Longman