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Tue, 03 Aug 2021 02:34:18 -0700 (PDT) MIME-Version: 1.0 References: <20210729125755.16871-1-linmiaohe@huawei.com> <20210729125755.16871-3-linmiaohe@huawei.com> <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com> <95629d91-6ae8-b445-e7fc-b51c888cad59@huawei.com> In-Reply-To: <95629d91-6ae8-b445-e7fc-b51c888cad59@huawei.com> From: Muchun Song Date: Tue, 3 Aug 2021 17:33:41 +0800 Message-ID: Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex To: Miaohe Lin Cc: Roman Gushchin , Michal Hocko , Johannes Weiner , Vladimir Davydov , Andrew Morton , Shakeel Butt , Matthew Wilcox , Alex Shi , Wei Yang , Linux Memory Management List , LKML , Cgroups Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: DA649B007ED0 Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=bytedance-com.20150623.gappssmtp.com header.s=20150623 header.b=jJSoKtxZ; dmarc=pass (policy=none) header.from=bytedance.com; spf=pass (imf19.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.214.180 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com X-Stat-Signature: 14i538adjdsidxtzqz4iacknj4k7d7ti X-HE-Tag: 1627983259-262900 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Aug 3, 2021 at 2:29 PM Miaohe Lin wrote: > > On 2021/8/3 11:40, Roman Gushchin wrote: > > On Sat, Jul 31, 2021 at 10:29:52AM +0800, 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 *m= emcg, 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 path= s. After > >>>> the proposed change it becomes obvious that the opposite is done her= e: 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 a= ny > >>> 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 mea= n is something like below=EF=BC=9F > >> > >> 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 *me= mcg, 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 wo= rkers. */ > >> - if (!mutex_trylock(&percpu_charge_mutex)) > >> + if (!atomic_inc_not_zero(&drain_all_stocks)) > >> return; > > > > It should work, but why not a simple atomic_cmpxchg(&drain_all_stocks, = 0, 1) and > > initialize it to 0? Maybe it's just my preference, but IMO (0, 1) is ea= sier > > to understand than (-1, 0) here. Not a strong opinion though, up to you= . > > > > I think this would improve the readability. What you mean is something li= ke below ? > > Many thanks. > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 616d1a72ece3..6210b1124929 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 drainer =3D ATOMIC_INIT(0); > > /* If someone's already draining, avoid adding running more worke= rs. */ > - if (!mutex_trylock(&percpu_charge_mutex)) > + if (atomic_cmpxchg(&drainer, 0, 1) !=3D 0) I'd like to use atomic_cmpxchg_acquire() here. > 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_set(&drainer, 0); So use atomic_set_release() here to cooperate with atomic_cmpxchg_acquire(). Thanks. > } > > > Thanks! > > . > > >