Re: [PATCH v3 4/8] memcg: replace stats_flush_lock with an atomic

From: Shakeel Butt <shakeelb@google.com>
To: "Michal Koutný" <mkoutny@suse.com>
Cc: Yosry Ahmed <yosryahmed@google.com>, 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>,
	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, Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH v3 4/8] memcg: replace stats_flush_lock with an atomic
Date: Tue, 4 Apr 2023 10:21:33 -0700	[thread overview]
Message-ID: <CALvZod7Ao-VmB4as+VHsR+awW1jmOA18uVM7qk21mVsXTOYC2A@mail.gmail.com> (raw)
In-Reply-To: <CALvZod5Y+quOS1XQvVBTvv7FRs3455j_79f0GoR+FqCFzbwkuA@mail.gmail.com>

On Tue, Apr 4, 2023 at 10:13 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Tue, Apr 4, 2023 at 9:53 AM Michal Koutný <mkoutny@suse.com> wrote:
> >
> > Hello.
> >
> > On Thu, Mar 30, 2023 at 07:17:57PM +0000, Yosry Ahmed <yosryahmed@google.com> wrote:
> > >  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;
> >
> > I'm curious about why this instead of
> >
> >         if (atomic_xchg(&stats_flush_ongoing, 1))
> >                 return;
> >
> > Is that some microarchitectural cleverness?
> >
>
> Yes indeed it is. Basically we want to avoid unconditional cache
> dirtying. This pattern is also used at other places in the kernel like
> qspinlock.

Oh also take a look at
https://lore.kernel.org/all/20230404052228.15788-1-feng.tang@intel.com/