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From: Kuniyuki Iwashima To: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt Cc: Muchun Song , Kuniyuki Iwashima , Kuniyuki Iwashima , linux-mm@kvack.org, cgroups@vger.kernel.org Content-Type: text/plain; charset="UTF-8" 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 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 Signed-off-by: Kuniyuki Iwashima --- 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