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

[Yosry Ahmed]

On Tue, Mar 28, 2023 at 12:34 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Mar 28, 2023 at 12:28 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> >
> > On Tue, Mar 28, 2023 at 11:53 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > >
> > [...]
> > > > > +     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?
> > >
> >
> > No, I don't think cmpxchg will be any different from xchg(). On x86,
> > the cmpxchg will always write to stats_flush_ongoing and depending on
> > the comparison result, it will either be 0 or 1 here.
> >
> > If you see the implementation of queued_spin_trylock(), it does the
> > same as well.
>
> Interesting. I thought cmpxchg by definition will compare first and
> only do the write if stats_flush_ongoing == 0 in this case.
>
> I thought queued_spin_trylock() was doing an atomic_read() first to
> avoid the LOCK instruction unnecessarily the lock is held by someone
> else.

Anyway, perhaps it's better to follow what queued_spin_trylock() is
doing, even if only to avoid locking the cache line unnecessarily.

(Although now that I think about it, I wonder why atomic_cmpxchg
doesn't do this by default, food for thought)