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

[Waiman Long <longman@redhat.com>]

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();
+	}
 }
 
 int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
@@ -3410,7 +3421,7 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 
 	ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
 	if (!ret && nr_bytes)
-		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes);
+		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, true);
 
 	css_put(&memcg->css);
 	return ret;
@@ -3418,7 +3429,7 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 
 void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
 {
-	refill_obj_stock(objcg, size);
+	refill_obj_stock(objcg, size, false);
 }
 
 #endif /* CONFIG_MEMCG_KMEM */
-- 
2.18.1