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

Shakeel Butt
2023-03-23T04:33:01Z

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
>