On Wed, Mar 29, 2023 at 8:58 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Tue 28-03-23 22:16:40, 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. > > Just a nit. Sounds more serious than it is actually. This would only > happen if compiler decides to split the write. Thanks for the note, Michal. I honestly quoted Johannes here as I do not have much expertise when it comes to this. I will add "if the compiler decides to split the write" to the commit log if I respin. > > > 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@xxxxxxxxxxx/ > > [2]https://lore.kernel.org/lkml/20210716212137.1391164-2-shakeelb@xxxxxxxxxx/ > > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > > > --- > > mm/memcontrol.c | 18 +++++++++++------- > > 1 file changed, 11 insertions(+), 7 deletions(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index ff39f78f962e..65750f8b8259 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -585,8 +585,8 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz) > > */ > > static void flush_memcg_stats_dwork(struct work_struct *w); > > static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork); > > -static DEFINE_SPINLOCK(stats_flush_lock); > > static DEFINE_PER_CPU(unsigned int, stats_updates); > > +static atomic_t stats_flush_ongoing = ATOMIC_INIT(0); > > static atomic_t stats_flush_threshold = ATOMIC_INIT(0); > > static u64 flush_next_time; > > > > @@ -636,15 +636,19 @@ 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)) > > + /* > > + * We always flush the entire tree, so concurrent flushers can just > > + * skip. This avoids a thundering herd problem on the rstat global lock > > + * from memcg flushers (e.g. reclaim, refault, etc). > > + */ > > + if (atomic_read(&stats_flush_ongoing) || > > + atomic_xchg(&stats_flush_ongoing, 1)) > > return; > > > > - flush_next_time = jiffies_64 + 2*FLUSH_TIME; > > + WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME); > > cgroup_rstat_flush_atomic(root_mem_cgroup->css.cgroup); > > atomic_set(&stats_flush_threshold, 0); > > - spin_unlock_irqrestore(&stats_flush_lock, flag); > > + atomic_set(&stats_flush_ongoing, 0); > > } > > > > void mem_cgroup_flush_stats(void) > > @@ -655,7 +659,7 @@ void mem_cgroup_flush_stats(void) > > > > void mem_cgroup_flush_stats_ratelimited(void) > > { > > - if (time_after64(jiffies_64, flush_next_time)) > > + if (time_after64(jiffies_64, READ_ONCE(flush_next_time))) > > mem_cgroup_flush_stats(); > > } > > > > -- > > 2.40.0.348.gf938b09366-goog > > -- > Michal Hocko > SUSE Labs
