Re: [RFC PATCH 2/7] memcg: do not disable interrupts when holding stats_flush_lock

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Mar 22, 2023 at 9:32 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Wed, Mar 22, 2023 at 9:00 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > The rstat flushing code was modified so that we do not disable interrupts
> > when we hold the global rstat lock. Do the same for stats_flush_lock on
> > the memcg side to avoid unnecessarily disabling interrupts throughout
> > flushing.
> >
> > Since the code exclusively uses trylock to acquire this lock, it should
> > be fine to hold from interrupt contexts or normal contexts without
> > disabling interrupts as a deadlock cannot occur. For interrupt contexts
> > we will return immediately without flushing anyway.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >  mm/memcontrol.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 5abffe6f8389..e0e92b38fa51 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -636,15 +636,17 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
> >
> >  static void __mem_cgroup_flush_stats(void)
> >  {
> > -       unsigned long flag;
> > -
> > -       if (!spin_trylock_irqsave(&stats_flush_lock, flag))
> > +       /*
> > +        * This lock can be acquired from interrupt context,
>
> How? What's the code path?

I believe through the charge/uncharge path we can do
memcg_check_events()->mem_cgroup_threshold()->mem_cgroup_usage()->mem_cgroup_flush_stats(),
right? I am assuming we can charge/uncharge memory in an interrupt
context.

Also the current code always disables interrupts before calling
memcg_check_events(), which made me suspect the percpu variables that
are modified by that call can also be modified in interrupt context.

>
> > but we only acquire
> > +        * using trylock so it should be fine as we cannot cause a deadlock.
> > +        */
> > +       if (!spin_trylock(&stats_flush_lock))
> >                 return;
> >
> >         flush_next_time = jiffies_64 + 2*FLUSH_TIME;
> >         cgroup_rstat_flush_irqsafe(root_mem_cgroup->css.cgroup);
> >         atomic_set(&stats_flush_threshold, 0);
> > -       spin_unlock_irqrestore(&stats_flush_lock, flag);
> > +       spin_unlock(&stats_flush_lock);
> >  }
> >
> >  void mem_cgroup_flush_stats(void)
> > --
> > 2.40.0.rc1.284.g88254d51c5-goog
> >