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

From: Waiman Long
Date: Mon Apr 19 2021 - 11:19:42 EST

On 4/19/21 11:00 AM, Shakeel Butt wrote:
On Sun, Apr 18, 2021 at 11:07 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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);
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);
+ }

+ 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.

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

I would recommend just rebase the patch series over the latest mm tree.

I see, I will rebase it to the latest mm tree.