linux-kernel - Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge

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Date:   Fri, 30 Jul 2021 08:50:02 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org,
        shakeelb@...gle.com, willy@...radead.org, alexs@...nel.org,
        richard.weiyang@...il.com, songmuchun@...edance.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex

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@...wei.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.
-- 
Michal Hocko
SUSE Labs