The patch titled
Subject: mm/memcg: improve refill_obj_stock() performance
has been removed from the -mm tree. Its filename was
mm-memcg-improve-refill_obj_stock-performance.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Waiman Long <longman@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Chris Down <chris@xxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Masayoshi Mizuma <msys.mizuma@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Roman Gushchin <guro@xxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
Cc: Xing Zhengjun <zhengjun.xing@xxxxxxxxxxxxxxx>
Cc: Yafang Shao <laoar.shao@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 48 +++++++++++++++++++++++++++++++++-------------
1 file changed, 35 insertions(+), 13 deletions(-)
--- a/mm/memcontrol.c~mm-memcg-improve-refill_obj_stock-performance
+++ a/mm/memcontrol.c
@@ -3157,10 +3157,12 @@ static bool obj_stock_flush_required(str
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_
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
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
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 */
_
Patches currently in -mm which might be from longman@xxxxxxxxxx are
