Re: [PATCH v1 5/9] memcg: replace stats_flush_lock with an atomic

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 28, 2023 at 06:16:34AM +0000, Yosry Ahmed wrote:
> As Johannes notes in [1], stats_flush_lock is currently used to:
> (a) Protect updated to stats_flush_threshold.
> (b) Protect updates to flush_next_time.
> (c) Serializes calls to cgroup_rstat_flush() based on those ratelimits.
> 
> However:
> 
> 1. stats_flush_threshold is already an atomic
> 
> 2. flush_next_time is not atomic. The writer is locked, but the reader
>    is lockless. If the reader races with a flush, you could see this:
> 
>                                         if (time_after(jiffies, flush_next_time))
>         spin_trylock()
>         flush_next_time = now + delay
>         flush()
>         spin_unlock()
>                                         spin_trylock()
>                                         flush_next_time = now + delay
>                                         flush()
>                                         spin_unlock()
> 
>    which means we already can get flushes at a higher frequency than
>    FLUSH_TIME during races. But it isn't really a problem.
> 
>    The reader could also see garbled partial updates, so it needs at
>    least READ_ONCE and WRITE_ONCE protection.
> 
> 3. Serializing cgroup_rstat_flush() calls against the ratelimit
>    factors is currently broken because of the race in 2. But the race
>    is actually harmless, all we might get is the occasional earlier
>    flush. If there is no delta, the flush won't do much. And if there
>    is, the flush is justified.
> 
> So the lock can be removed all together. However, the lock also served
> the purpose of preventing a thundering herd problem for concurrent
> flushers, see [2]. Use an atomic instead to serve the purpose of
> unifying concurrent flushers.
> 
> [1]https://lore.kernel.org/lkml/20230323172732.GE739026@cmpxchg.org/
> [2]https://lore.kernel.org/lkml/20210716212137.1391164-2-shakeelb@google.com/
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

With Shakeel's suggestion:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>