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

Michal Hocko
2021-07-30T06:50:02Z

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.
--
Michal Hocko
SUSE Labs