On Sun, Jun 24, 2012 at 06:32:58PM +0800, Wanpeng Li wrote:

On Sun, Jun 24, 2012 at 12:19:48PM +0200, Johannes Weiner wrote:

quoted

On Sun, Jun 24, 2012 at 06:08:26PM +0800, Wanpeng Li wrote:

quoted

On Sun, Jun 24, 2012 at 11:46:14AM +0200, Johannes Weiner wrote:

quoted

On Sun, Jun 24, 2012 at 10:16:09AM +0800, Wanpeng Li wrote:

quoted

From: Wanpeng Li <redacted>

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 <redacted>
---
 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);

The only way excessive stock can build up is if the charging task gets
rescheduled, after trying to consume stock a few lines above, to a cpu
it was running on when it built up stock in the past.

   consume_stock()
     memcg != stock->cached:
       return false
   do_charge()
   <reschedule>
   refill_stock()
     memcg == stock->cached:
       stock->nr_pages += nr_pages

__mem_cgroup_try_charge() {
	unsigned int batch = max(CHARGE_BATCH, nr_pages);
	[...]
	mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
	[...]
	if(batch > nr_pages)
		refill_stock(memcg, batch - nr_pages);
}

Consider this scenario, If one task wants to charge nr_pages = 1,
then batch = max(32,1) = 32, this time 31 excess charges 
will be charged in mem_cgroup_do_charge and then add to stock by
refill_stock. Generally there are many tasks in one memory cgroup and 
maybe charges frequency. In this situation, limit will reach soon, 
and cause mem_cgroup_reclaim to call try_to_free_mem_cgroup_pages.

But the stock is not a black hole that gets built up for giggles!  The
next time the processes want to charge a page on this cpu, they will
consume it from the stock.  Not add more pages to it.  Look at where
consume_stock() is called.

if(nr_pages == 1 && consume_stock(memcg))
	goto done;

Only when charge one page will call consume_stock. You can see the codes
in mem_cgroup_charge_common() which also call __mem_cgroup_try_charge, 
when both transparent huge and hugetlbfs pages, nr_pages will larger than 1.

In which case, nr_pages will be bigger than CHARGE_BATCH, in which
case batch equals nr_pages, in which case stock won't be refilled:

  unsigned int batch = max(CHARGE_BATCH, nr_pages);
  ...
  if (batch > nr_pages)
    refill_stock(memcg, batch - nr_pages);

We could maybe make this

  if (nr_pages == 1 && batch > nr_pages)
    ...

for clarity, but it won't make a behavioural difference.

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