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

[Shakeel Butt]

On Tue, Apr 4, 2023 at 9:53 AM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> Hello.
>
> On Thu, Mar 30, 2023 at 07:17:57PM +0000, Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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.