Re: [PATCH 1/3] mm: page_counter: remove unneeded atomic ops for low/min

[Shakeel Butt <shakeelb@google.com>]

On Mon, Aug 22, 2022 at 3:18 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 22-08-22 11:55:33, Michal Hocko wrote:
> > On Mon 22-08-22 00:17:35, Shakeel Butt wrote:
> [...]
> > > diff --git a/mm/page_counter.c b/mm/page_counter.c
> > > index eb156ff5d603..47711aa28161 100644
> > > --- a/mm/page_counter.c
> > > +++ b/mm/page_counter.c
> > > @@ -17,24 +17,23 @@ static void propagate_protected_usage(struct page_counter *c,
> > >                                   unsigned long usage)
> > >  {
> > >     unsigned long protected, old_protected;
> > > -   unsigned long low, min;
> > >     long delta;
> > >
> > >     if (!c->parent)
> > >             return;
> > >
> > > -   min = READ_ONCE(c->min);
> > > -   if (min || atomic_long_read(&c->min_usage)) {
> > > -           protected = min(usage, min);
> > > +   protected = min(usage, READ_ONCE(c->min));
> > > +   old_protected = atomic_long_read(&c->min_usage);
> > > +   if (protected != old_protected) {
> >
> > I have to cache that code back into brain. It is really subtle thing and
> > it is not really obvious why this is still correct. I will think about
> > that some more but the changelog could help with that a lot.
>
> OK, so the this patch will be most useful when the min > 0 && min <
> usage because then the protection doesn't really change since the last
> call. In other words when the usage grows above the protection and your
> workload benefits from this change because that happens a lot as only a
> part of the workload is protected. Correct?

Yes, that is correct. I hope the experiment setup is clear now.

>
> Unless I have missed anything this shouldn't break the correctness but I
> still have to think about the proportional distribution of the
> protection because that adds to the complexity here.

The patch is not changing any semantics. It is just removing an
unnecessary atomic xchg() for a specific scenario (min > 0 && min <
usage). I don't think there will be any change related to proportional
distribution of the protection.