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

From: Yosry Ahmed <yosryahmed@google.com>
To: Shakeel Butt <shakeelb@google.com>
Cc: "Tejun Heo" <tj@kernel.org>, "Josef Bacik" <josef@toxicpanda.com>,
	"Jens Axboe" <axboe@kernel.dk>,
	"Zefan Li" <lizefan.x@bytedance.com>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Roman Gushchin" <roman.gushchin@linux.dev>,
	"Muchun Song" <muchun.song@linux.dev>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Michal Koutný" <mkoutny@suse.com>,
	"Vasily Averin" <vasily.averin@linux.dev>,
	cgroups@vger.kernel.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	bpf@vger.kernel.org
Subject: Re: [PATCH v1 5/9] memcg: replace stats_flush_lock with an atomic
Date: Tue, 28 Mar 2023 11:52:50 -0700	[thread overview]
Message-ID: <CAJD7tkYo=CeXJPUi_KxjzC0QCxC2qd_J2_FQi_aXh7svD8u60A@mail.gmail.com> (raw)
In-Reply-To: <20230328141523.txyhl7wt7wtvssea@google.com>

On Tue, Mar 28, 2023 at 7:15 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Tue, Mar 28, 2023 at 06:16:34AM +0000, Yosry Ahmed wrote:
> [...]
> > @@ -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 +636,18 @@ 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_xchg(&stats_flush_ongoing, 1))
>
> Have you profiled this? I wonder if we should replace the above with
>
>         if (atomic_read(&stats_flush_ongoing) || atomic_xchg(&stats_flush_ongoing, 1))

I profiled the entire series with perf and I haven't noticed a notable
difference between before and after the patch series -- but maybe some
specific access patterns cause a regression, not sure.

Does an atomic_cmpxchg() satisfy the same purpose? it's easier to read
/ more concise I guess.

Something like

    if (atomic_cmpxchg(&stats_flush_ongoing, 0, 1))

WDYT?

>
> to not always dirty the cacheline. This would not be an issue if there
> is no cacheline sharing but I suspect percpu stats_updates is sharing
> the cacheline with it and may cause false sharing with the parallel stat
> updaters (updaters only need to read the base percpu pointer).
>
> Other than that the patch looks good.