[PATCH] mm/memcg: add MAX_CHARGE_BATCH to limit unnecessary charge overhead

[Wanpeng Li <liwp.linux@gmail.com>]

From: Wanpeng Li <liwp@linux.vnet.ibm.com>

Since exceeded unused cached charges would add pressure to
mem_cgroup_do_charge, more overhead would burn cpu cycles when
mem_cgroup_do_charge cause page reclaim or even OOM be triggered
just for such exceeded unused cached charges. Add MAX_CHARGE_BATCH
to limit max cached charges.

Signed-off-by: Wanpeng Li <liwp.linux@gmail.com>
---
 mm/memcontrol.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0e092eb..1ff317a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1954,6 +1954,14 @@ void mem_cgroup_update_page_stat(struct page *page,
  * TODO: maybe necessary to use big numbers in big irons.
  */
 #define CHARGE_BATCH	32U
+
+/*
+ * Max size of charge stock. Since exceeded unused cached charges would
+ * add pressure to mem_cgroup_do_charge which will cause page reclaim or
+ * even oom be triggered.
+ */
+#define MAX_CHARGE_BATCH 1024U
+
 struct memcg_stock_pcp {
 	struct mem_cgroup *cached; /* this never be root cgroup */
 	unsigned int nr_pages;
@@ -2250,6 +2258,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	unsigned int batch = max(CHARGE_BATCH, nr_pages);
 	int nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
 	struct mem_cgroup *memcg = NULL;
+	struct memcg_stock_pcp *stock;
 	int ret;
 
 	/*
@@ -2320,6 +2329,13 @@ again:
 		rcu_read_unlock();
 	}
 
+	stock = &get_cpu_var(memcg_stock);
+	if (memcg == stock->cached && stock->nr_pages) {
+		if (stock->nr_pages > MAX_CHARGE_BATCH)
+			batch = nr_pages;
+	}
+	put_cpu_var(memcg_stock);
+
 	do {
 		bool oom_check;
 
-- 
1.7.9.5

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