From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.4 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3A713C4338F for ; Sat, 31 Jul 2021 02:29:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1CA9960EB2 for ; Sat, 31 Jul 2021 02:29:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236802AbhGaCaC (ORCPT ); Fri, 30 Jul 2021 22:30:02 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:7769 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231335AbhGaCaB (ORCPT ); Fri, 30 Jul 2021 22:30:01 -0400 Received: from dggeme703-chm.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4Gc7P62Cc9zYjb7; Sat, 31 Jul 2021 10:23:54 +0800 (CST) Received: from [10.174.178.209] (10.174.178.209) by dggeme703-chm.china.huawei.com (10.1.199.99) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2176.2; Sat, 31 Jul 2021 10:29:53 +0800 Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex To: Michal Hocko , Roman Gushchin CC: , , , , , , , , , , References: <20210729125755.16871-1-linmiaohe@huawei.com> <20210729125755.16871-3-linmiaohe@huawei.com> From: Miaohe Lin Message-ID: <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com> Date: Sat, 31 Jul 2021 10:29:52 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.178.209] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggeme703-chm.china.huawei.com (10.1.199.99) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/7/30 14:50, Michal Hocko wrote: > On Thu 29-07-21 20:06:45, Roman Gushchin wrote: >> On Thu, Jul 29, 2021 at 08:57:52PM +0800, Miaohe Lin wrote: >>> Since percpu_charge_mutex is only used inside drain_all_stock(), we can >>> narrow the scope of percpu_charge_mutex by moving it here. >>> >>> Signed-off-by: Miaohe Lin >>> --- >>> mm/memcontrol.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>> index 6580c2381a3e..a03e24e57cd9 100644 >>> --- a/mm/memcontrol.c >>> +++ b/mm/memcontrol.c >>> @@ -2050,7 +2050,6 @@ struct memcg_stock_pcp { >>> #define FLUSHING_CACHED_CHARGE 0 >>> }; >>> static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock); >>> -static DEFINE_MUTEX(percpu_charge_mutex); >>> >>> #ifdef CONFIG_MEMCG_KMEM >>> static void drain_obj_stock(struct obj_stock *stock); >>> @@ -2209,6 +2208,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) >>> */ >>> static void drain_all_stock(struct mem_cgroup *root_memcg) >>> { >>> + static DEFINE_MUTEX(percpu_charge_mutex); >>> int cpu, curcpu; >> >> It's considered a good practice to protect data instead of code paths. After >> the proposed change it becomes obvious that the opposite is done here: the mutex >> is used to prevent a simultaneous execution of the code of the drain_all_stock() >> function. > > The purpose of the lock was indeed to orchestrate callers more than any > data structure consistency. > >> Actually we don't need a mutex here: nobody ever sleeps on it. So I'd replace >> it with a simple atomic variable or even a single bitfield. Then the change will >> be better justified, IMO. > > Yes, mutex can be replaced by an atomic in a follow up patch. > Thanks for both of you. It's a really good suggestion. What do you mean is something like below？ diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 616d1a72ece3..508a96e80980 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2208,11 +2208,11 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) */ static void drain_all_stock(struct mem_cgroup *root_memcg) { - static DEFINE_MUTEX(percpu_charge_mutex); int cpu, curcpu; + static atomic_t drain_all_stocks = ATOMIC_INIT(-1); /* If someone's already draining, avoid adding running more workers. */ - if (!mutex_trylock(&percpu_charge_mutex)) + if (!atomic_inc_not_zero(&drain_all_stocks)) return; /* * Notify other cpus that system-wide "drain" is running @@ -2244,7 +2244,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) } } put_cpu(); - mutex_unlock(&percpu_charge_mutex); + atomic_dec(&drain_all_stocks); } static int memcg_hotplug_cpu_dead(unsigned int cpu)