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Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex
To:     Roman Gushchin <guro@fb.com>
CC:     Michal Hocko <mhocko@suse.com>, <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>
References: <20210729125755.16871-1-linmiaohe@huawei.com>
 <20210729125755.16871-3-linmiaohe@huawei.com> <YQNsxVPsRSBZcfGG@carbon.lan>
 <YQOhGs3k9rHx3mmT@dhcp22.suse.cz>
 <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com>
 <YQi6lOT6j2DtOGlT@carbon.dhcp.thefacebook.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <95629d91-6ae8-b445-e7fc-b51c888cad59@huawei.com>
Date:   Tue, 3 Aug 2021 14:29:13 +0800
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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)
                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);
 }

> Thanks!
> .
>