[PATCH v2 1/6] memcg: Remove unnecessary READ_ONCE() under memcg_stock.lock.

[Kuniyuki Iwashima <kuniyu@google.com>]

this_cpu_ptr(&memcg_stock) is always modified under
local_lock(&memcg_stock.lock).

Unlike is_memcg_drain_needed() under RCU, consume_stock(),
drain_stock(), and refill_stock() do not need READ_ONCE().

Especially, in refill_stock(), READ_ONCE() is used twice for
stock->cached[i], which forces the compiler to emit two volatile
loads from stock->cached[i].

Let's remove the unnecessary READ_ONCE() and eliminate the
redundant loads in refill_stock():

Before:

  cached = READ_ONCE(stock->cached[i]);
   4c 8b 80 88 d7 bd 84 	movq	-0x7b422878(%rax), %r8
  if (memcg == READ_ONCE(stock->cached[i])) {
   48 3b 98 88 d7 bd 84 	cmpq	-0x7b422878(%rax), %rbx
   0f 84 86 01 00 00    	je	0xffffffff817d01e0 <refill_stock+0x210>

After:

  cached = stock->cached[i];
   4c 8b 88 88 d7 bd 84 	movq	-0x7b422878(%rax), %r9
  if (memcg == stock->cached[i]) {
   4c 39 cb             	cmpq	%r9, %rbx
   0f 84 7a 01 00 00    	je	0xffffffff817d01d0 <refill_stock+0x200>

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 mm/memcontrol.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 84f4a8fd4d16..8a401550dfaa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2164,10 +2164,10 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	stock = this_cpu_ptr(&memcg_stock);
 
 	for (i = 0; i < NR_MEMCG_STOCK; ++i) {
-		if (memcg != READ_ONCE(stock->cached[i]))
+		if (memcg != stock->cached[i])
 			continue;
 
-		stock_pages = READ_ONCE(stock->nr_pages[i]);
+		stock_pages = stock->nr_pages[i];
 		if (stock_pages >= nr_pages) {
 			stock_pages -= nr_pages;
 			WRITE_ONCE(stock->nr_pages[i], stock_pages);
@@ -2197,13 +2197,13 @@ static void memcg_uncharge(struct mem_cgroup *memcg, unsigned int nr_pages)
  */
 static void drain_stock(struct memcg_stock_pcp *stock, int i)
 {
-	struct mem_cgroup *old = READ_ONCE(stock->cached[i]);
+	struct mem_cgroup *old = stock->cached[i];
 	uint8_t stock_pages;
 
 	if (!old)
 		return;
 
-	stock_pages = READ_ONCE(stock->nr_pages[i]);
+	stock_pages = stock->nr_pages[i];
 	if (stock_pages) {
 		memcg_uncharge(old, stock_pages);
 		WRITE_ONCE(stock->nr_pages[i], 0);
@@ -2284,11 +2284,11 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 	stock = this_cpu_ptr(&memcg_stock);
 	for (i = 0; i < NR_MEMCG_STOCK; ++i) {
-		cached = READ_ONCE(stock->cached[i]);
+		cached = stock->cached[i];
 		if (!cached && empty_slot == -1)
 			empty_slot = i;
-		if (memcg == READ_ONCE(stock->cached[i])) {
-			stock_pages = READ_ONCE(stock->nr_pages[i]) + nr_pages;
+		if (memcg == stock->cached[i]) {
+			stock_pages = stock->nr_pages[i] + nr_pages;
 			if (stock_pages > MEMCG_STOCK_HIGH) {
 				memcg_uncharge(memcg,
 					       stock_pages - MEMCG_STOCK_LOW);
-- 
2.56.0.313.g2479322e68-goog