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

Roman Gushchin
2021-07-30T03:06:45Z

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.

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!