Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex

Miaohe Lin
2021-08-03T06:29:13Z

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-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>>>>> ---
>>>>> 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 belowï¼Ÿ
>>
>> 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!
> .
>