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

[Shakeel Butt <shakeelb@google.com>]

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?

> 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
>