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

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On 4/19/21 2:06 AM, Muchun Song wrote:
On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxx>
---
  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@xxxxxxxxxxxxx/

In the above patchset, we introduce obj_cgroup_uncharge_pages to
uncharge some pages from object cgroup. You can use this safe
API.

Thanks for the comment. Will update my patch with call to obj_cgroup_uncharge_pages().

Cheers,
Longman




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