[PATCH-next v5 3/4] mm/memcg: Improve refill_obj_stock() performance

[Waiman Long]

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().

A multithreaded kmalloc+kfree microbenchmark on a 2-socket 48-core
96-thread x86-64 system with 96 testing threads were run.  Before this
patch, the total number of kilo kmalloc+kfree operations done for a 4k
large object by all the testing threads per second were 4,304 kops/s
(cgroup v1) and 8,478 kops/s (cgroup v2). After applying this patch, the
number were 4,731 (cgroup v1) and 418,142 (cgroup v2) respectively. This
represents a performance improvement of 1.10X (cgroup v1) and 49.3X
(cgroup v2).

Signed-off-by: Waiman Long <long...@redhat.com>
---
 mm/memcontrol.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 292b4783b1a7..2f87d0b05092 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3153,10 +3153,12 @@ 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)
 {
        struct memcg_stock_pcp *stock;
        unsigned long flags;
+       unsigned int nr_pages = 0;
 
        local_irq_save(flags);
 
@@ -3165,14 +3167,20 @@ static void refill_obj_stock(struct obj_cgroup *objcg, 
unsigned int nr_bytes)
                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 = atomic_read(&objcg->nr_charged_bytes)
+                               ? atomic_xchg(&objcg->nr_charged_bytes, 0) : 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);
+       }
 
        local_irq_restore(flags);
+
+       if (nr_pages)
+               obj_cgroup_uncharge_pages(objcg, nr_pages);
 }
 
 int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
@@ -3201,14 +3209,14 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t 
gfp, size_t size)
 
        ret = obj_cgroup_charge_pages(objcg, gfp, nr_pages);
        if (!ret && nr_bytes)
-               refill_obj_stock(objcg, PAGE_SIZE - nr_bytes);
+               refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, true);
 
        return ret;
 }
 
 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