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dkim=pass header.d=fb.com header.s=facebook header.b="rh24/XUl"; dmarc=pass (policy=reject) header.from=fb.com; spf=pass (imf12.hostedemail.com: domain of "prvs=78499dd93f=guro@fb.com" designates 67.231.145.42 as permitted sender) smtp.mailfrom="prvs=78499dd93f=guro@fb.com" X-Stat-Signature: itiukbh1fr1tesjtwfytb9zgpjycis6h X-HE-Tag: 1627962032-174335 Content-Transfer-Encoding: quoted-printable 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 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); > >>> =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 *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. > >=20 > > The purpose of the lock was indeed to orchestrate callers more than a= ny > > 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= change 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= is 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 *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; > + static atomic_t drain_all_stocks =3D ATOMIC_INIT(-1); >=20 > /* If someone's already draining, avoid adding running more wor= kers. */ > - 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 easi= er to understand than (-1, 0) here. Not a strong opinion though, up to you. Thanks!