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.
