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

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Aug 3, 2021 at 2:29 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> 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@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.
> >
>
> 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)

I'd like to use atomic_cmpxchg_acquire() here.

>                 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);

So use atomic_set_release() here to cooperate with
atomic_cmpxchg_acquire().

Thanks.

>  }
>
> > Thanks!
> > .
> >
>