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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 440C7C4338F for ; Mon, 2 Aug 2021 06:49:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 24E8661057 for ; Mon, 2 Aug 2021 06:49:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232431AbhHBGtb (ORCPT ); Mon, 2 Aug 2021 02:49:31 -0400 Received: from smtp-out2.suse.de ([195.135.220.29]:36690 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229734AbhHBGta (ORCPT ); Mon, 2 Aug 2021 02:49:30 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id A13871FF2C; Mon, 2 Aug 2021 06:49:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1627886960; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=q7PGCIa5dKr+zd9QXo1w6BYL7jP8T2ubQzOp7fxIYBo=; b=HU962FsCvbDVraAaI8paM+qVfe5IewCn44+dP/CX81ZlPfpNkBA8DsBLvPSoYPFZoqjIiN xNW7R0UwCkOFg8/qZuytNpEVqMkSnSw/rnXR4hQuuwbd3VXQhH/3XeLGwPZFQH5L/4+NIz E1gqlLeGGJ84o7CSyorpMjlhAUSJQC0= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 73312A3B85; Mon, 2 Aug 2021 06:49:20 +0000 (UTC) Date: Mon, 2 Aug 2021 08:49:04 +0200 From: Michal Hocko To: Miaohe Lin Cc: Roman Gushchin , hannes@cmpxchg.org, vdavydov.dev@gmail.com, akpm@linux-foundation.org, shakeelb@google.com, willy@infradead.org, alexs@kernel.org, richard.weiyang@gmail.com, songmuchun@bytedance.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex Message-ID: References: <20210729125755.16871-1-linmiaohe@huawei.com> <20210729125755.16871-3-linmiaohe@huawei.com> <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 31-07-21 10:29:52, Miaohe Lin wrote: > 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); Yes this would work. I would just s@drain_all_stocks@drainers@ or something similar to better express the intention. > } > > static int memcg_hotplug_cpu_dead(unsigned int cpu) -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex Date: Mon, 2 Aug 2021 08:49:04 +0200 Message-ID: References: <20210729125755.16871-1-linmiaohe@huawei.com> <20210729125755.16871-3-linmiaohe@huawei.com> <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1627886960; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=q7PGCIa5dKr+zd9QXo1w6BYL7jP8T2ubQzOp7fxIYBo=; b=HU962FsCvbDVraAaI8paM+qVfe5IewCn44+dP/CX81ZlPfpNkBA8DsBLvPSoYPFZoqjIiN xNW7R0UwCkOFg8/qZuytNpEVqMkSnSw/rnXR4hQuuwbd3VXQhH/3XeLGwPZFQH5L/4+NIz E1gqlLeGGJ84o7CSyorpMjlhAUSJQC0= Content-Disposition: inline In-Reply-To: <4a3c23c4-054c-2896-29c5-8cf9a4deee98-hv44wF8Li93QT0dZR+AlfA@public.gmane.org> List-ID: Content-Type: text/plain; charset="windows-1252" To: Miaohe Lin Cc: Roman Gushchin , hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org, alexs-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, richard.weiyang-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On Sat 31-07-21 10:29:52, Miaohe Lin wrote: > 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 c= an > >>> 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); > >>> =20 > >>> #ifdef CONFIG_MEMCG_KMEM > >>> static void drain_obj_stock(struct obj_stock *stock); > >>> @@ -2209,6 +2208,7 @@ static void refill_stock(struct mem_cgroup *mem= cg, 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_a= ll_stock() > >> function. > >=20 > > The purpose of the lock was indeed to orchestrate callers more than any > > data structure consistency. > > =20 > >> 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 c= hange will > >> be better justified, IMO. > >=20 > > Yes, mutex can be replaced by an atomic in a follow up patch. > >=20 >=20 > Thanks for both of you. It's a really good suggestion. What do you mean i= s something like below=EF=BC=9F >=20 > 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 =3D ATOMIC_INIT(-1); > /* If someone's already draining, avoid adding running more worke= rs. */ > - 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); Yes this would work. I would just s@drain_all_stocks@drainers@ or something similar to better express the intention. > } >=20 > static int memcg_hotplug_cpu_dead(unsigned int cpu) --=20 Michal Hocko SUSE Labs