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

Miaohe Lin
2021-07-31T02:29:52Z

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;
/*
* 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_dec(&drain_all_stocks);
}

static int memcg_hotplug_cpu_dead(unsigned int cpu)