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From: Muchun Song <songmuchun@bytedance.com>
To: Miaohe Lin <linmiaohe@huawei.com>
Cc: Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@suse.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Shakeel Butt <shakeelb@google.com>,
	Matthew Wilcox <willy@infradead.org>, Alex Shi <alexs@kernel.org>,
	Wei Yang <richard.weiyang@gmail.com>,
	Linux Memory Management List <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Cgroups <cgroups@vger.kernel.org>
Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex
Date: Tue, 3 Aug 2021 17:33:41 +0800	[thread overview]
Message-ID: <CAMZfGtUChsJO1UrgmP6M274UwiHap0_yzCBL+mDq1OosP7JNsA@mail.gmail.com> (raw)
In-Reply-To: <95629d91-6ae8-b445-e7fc-b51c888cad59@huawei.com>

On Tue, Aug 3, 2021 at 2:29 PM Miaohe Lin <linmiaohe@huawei.com> 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 <linmiaohe@huawei.com>
> >>>>> ---
> >>>>>  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;
> >
> > 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 easier
> > 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 like 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 = ATOMIC_INIT(0);
>
>         /* If someone's already draining, avoid adding running more workers. */
> -       if (!mutex_trylock(&percpu_charge_mutex))
> +       if (atomic_cmpxchg(&drainer, 0, 1) != 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!
> > .
> >
>

  parent reply	other threads:[~2021-08-03  9:34 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 12:57 [PATCH 0/5] Cleanups and fixup for memcontrol Miaohe Lin
2021-07-29 12:57 ` [PATCH 1/5] mm, memcg: remove unused functions Miaohe Lin
2021-07-29 14:07   ` Shakeel Butt
2021-07-30  2:39   ` Muchun Song
2021-07-30  2:57   ` Roman Gushchin
2021-07-30  6:45   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex Miaohe Lin
2021-07-30  2:42   ` Muchun Song
2021-07-30  3:06   ` Roman Gushchin
2021-07-30  6:50     ` Michal Hocko
2021-07-31  2:29       ` Miaohe Lin
2021-08-02  6:49         ` Michal Hocko
2021-08-02  9:54           ` Miaohe Lin
2021-08-03  3:40         ` Roman Gushchin
2021-08-03  6:29           ` Miaohe Lin
2021-08-03  7:11             ` Michal Hocko
2021-08-03  7:13               ` Roman Gushchin
2021-08-03  7:27                 ` Michal Hocko
2021-08-03  9:33             ` Muchun Song [this message]
2021-08-03 10:50               ` Miaohe Lin
2021-08-03 14:15       ` Johannes Weiner
2021-08-04  8:20         ` Michal Hocko
2021-08-05  1:44           ` Miaohe Lin
2021-07-30  6:46   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 3/5] mm, memcg: save some atomic ops when flush is already true Miaohe Lin
2021-07-29 14:40   ` Shakeel Butt
2021-07-30  2:37   ` Muchun Song
2021-07-30  3:07   ` Roman Gushchin
2021-07-30  6:51   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup_init() Miaohe Lin
2021-07-29 13:52   ` Matthew Wilcox
2021-07-30  1:50     ` Miaohe Lin
2021-07-30  3:12   ` Roman Gushchin
2021-07-30  6:29     ` Miaohe Lin
2021-07-30  6:44     ` Michal Hocko
2021-07-31  2:05       ` Miaohe Lin
2021-08-02  6:43         ` Michal Hocko
2021-08-02 10:00           ` Miaohe Lin
2021-08-02 10:42             ` Michal Hocko
2021-08-02 11:18               ` Miaohe Lin
2021-07-29 12:57 ` [PATCH 5/5] mm, memcg: always call __mod_node_page_state() with preempt disabled Miaohe Lin
2021-07-29 14:39   ` Shakeel Butt
2021-07-30  1:52     ` Miaohe Lin
2021-07-30  2:33       ` [External] " Muchun Song
2021-07-30  2:46         ` Miaohe Lin

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