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

[Roman Gushchin <guro@fb.com>]

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.

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.

Thanks!