mm/memcg: improve refill_obj_stock() performance

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
allow_uncharge flag is added to refill_obj_stock() which will be set to
false when called from obj_cgroup_charge() so that an uncharge_pages()
call won't be issued right after a charge_pages() call unless the objcg
changes.

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).

Link: https://lkml.kernel.org/r/20210506150007.16288-4-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Chris Down <chris@chrisdown.name>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Masayoshi Mizuma <msys.mizuma@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Cc: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b462458..17d38c7 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3157,10 +3157,12 @@ 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 allow_uncharge)
 {
        struct memcg_stock_pcp *stock;
        unsigned long flags;
+       unsigned int nr_pages = 0;
 
        local_irq_save(flags);
 
@@ -3169,14 +3171,21 @@ 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;
+               allow_uncharge = true;  /* Allow uncharge when objcg changes */
        }
        stock->nr_bytes += nr_bytes;
 
-       if (stock->nr_bytes > PAGE_SIZE)
-               drain_obj_stock(stock);
+       if (allow_uncharge && (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)
@@ -3188,14 +3197,27 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
                return 0;
 
        /*
-        * In theory, memcg->nr_charged_bytes can have enough
+        * In theory, objcg->nr_charged_bytes can have enough
         * pre-charged bytes to satisfy the allocation. However,
-        * flushing memcg->nr_charged_bytes requires two atomic
-        * operations, and memcg->nr_charged_bytes can't be big,
-        * so it's better to ignore it and try grab some new pages.
-        * memcg->nr_charged_bytes will be flushed in
-        * refill_obj_stock(), called from this function or
-        * independently later.
+        * flushing objcg->nr_charged_bytes requires two atomic
+        * operations, and objcg->nr_charged_bytes can't be big.
+        * The shared objcg->nr_charged_bytes can also become a
+        * performance bottleneck if all tasks of the same memcg are
+        * trying to update it. So it's better to ignore it and try
+        * grab some new pages. The stock's nr_bytes will be flushed to
+        * objcg->nr_charged_bytes later on when objcg changes.
+        *
+        * The stock's nr_bytes may contain enough pre-charged bytes
+        * to allow one less page from being charged, but we can't rely
+        * on the pre-charged bytes not being changed outside of
+        * consume_obj_stock() or refill_obj_stock(). So ignore those
+        * pre-charged bytes as well when charging pages. To avoid a
+        * page uncharge right after a page charge, we set the
+        * allow_uncharge flag to false when calling refill_obj_stock()
+        * to temporarily allow the pre-charged bytes to exceed the page
+        * size limit. The maximum reachable value of the pre-charged
+        * bytes is (sizeof(object) + PAGE_SIZE - 2) if there is no data
+        * race.
         */
        nr_pages = size >> PAGE_SHIFT;
        nr_bytes = size & (PAGE_SIZE - 1);
@@ -3205,14 +3227,14 @@ 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, false);
 
        return ret;
 }
 
 void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
 {
-       refill_obj_stock(objcg, size);
+       refill_obj_stock(objcg, size, true);
 }
 
 #endif /* CONFIG_MEMCG_KMEM */