Re: [PATCH v2 04/13] memcg: Make it possible to use the stock for more than one page.

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On 03/10/2012 12:39 AM, Suleiman Souhlal wrote:
Signed-off-by: Suleiman Souhlal<suleiman@xxxxxxxxxx>
---
  mm/memcontrol.c |   18 +++++++++---------
  1 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6fbb438..f605100 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1965,19 +1965,19 @@ static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
  static DEFINE_MUTEX(percpu_charge_mutex);

  /*
- * Try to consume stocked charge on this cpu. If success, one page is consumed
- * from local stock and true is returned. If the stock is 0 or charges from a
- * cgroup which is not current target, returns false. This stock will be
- * refilled.
+ * Try to consume stocked charge on this cpu. If success, nr_pages pages are
+ * consumed from local stock and true is returned. If the stock is 0 or
+ * charges from a cgroup which is not current target, returns false.
+ * This stock will be refilled.
   */
-static bool consume_stock(struct mem_cgroup *memcg)
+static bool consume_stock(struct mem_cgroup *memcg, int nr_pages)
  {
  	struct memcg_stock_pcp *stock;
  	bool ret = true;

  	stock =&get_cpu_var(memcg_stock);
-	if (memcg == stock->cached&&  stock->nr_pages)
-		stock->nr_pages--;
+	if (memcg == stock->cached&&  stock->nr_pages>= nr_pages)
+		stock->nr_pages -= nr_pages;
  	else /* need to call res_counter_charge */
  		ret = false;
  	put_cpu_var(memcg_stock);
@@ -2290,7 +2290,7 @@ again:
  		VM_BUG_ON(css_is_removed(&memcg->css));
  		if (mem_cgroup_is_root(memcg))
  			goto done;
-		if (nr_pages == 1&&  consume_stock(memcg))
+		if (consume_stock(memcg, nr_pages))
  			goto done;
  		css_get(&memcg->css);
  	} else {
@@ -2315,7 +2315,7 @@ again:
  			rcu_read_unlock();
  			goto done;
  		}
-		if (nr_pages == 1&&  consume_stock(memcg)) {
+		if (consume_stock(memcg, nr_pages)) {
  			/*
  			 * It seems dagerous to access memcg without css_get().
  			 * But considering how consume_stok works, it's not

This patch itself is fine in what it wants to achieve.
But it made me think:

We'll jump into the stock code which makes user allocation faster.
but we're not getting the benefit of it when we're accounting kmem.
since we're allocating to both res_counters, we're actually defeating it altogether, since we now have to go to the global poll *everytime* (for memcg->kmem).

It would make a whole lot more sense to have the stock code moved to the
res_counter. We're now starting to have more users of that anyway, so
a common implementation makes sense.

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