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

[Roman Gushchin <guro@fb.com>]

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.

Thanks!