On Mon, Jul 22, 2024 at 3:47 PM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> On 7/22/24 15:30, David Finkel wrote:
> >>> diff --git a/mm/page_counter.c b/mm/page_counter.c
> >>> index db20d6452b71..40d5f4990218 100644
> >>> --- a/mm/page_counter.c
> >>> +++ b/mm/page_counter.c
> >>> @@ -82,6 +82,8 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
> >>> */
> >>> if (new > READ_ONCE(c->watermark))
> >>> WRITE_ONCE(c->watermark, new);
> >>> + if (new > READ_ONCE(c->local_watermark))
> >>> + WRITE_ONCE(c->local_watermark, new);
> >> Hm, can't we have a single comparison on the hot path?
> >> Also, we read and write c->local_watermark speculatively here, Idk if it's still
> >> acceptable with an ability to reset watermarks "locally". Maybe it is, but
> >> it definitely deserves at least a comment with an explanation.
> > Unfortunately, since the two watermarks may be reset at different
> > times I don't think we
> > can consolidate.
> > e.g. I think that if the usage peaked, dropped down a bit and then was
> > going back
> > up again when the "local_watermark" was reset, we'll continue only
> > bumping local_watermark,
> > but we don't want to touch "watermark" until we hit that watermark again.
> If we make page_counter_reset_watermark() reset the local_watermark as well,
> we can guarantee "local_watermark <= watermark" and wrap one check inside
> the other.
>
> if (new > READ_ONCE(c->local_watermark)) {
> WRITE_ONCE(c->local_watermark, new);
> if (new > READ_ONCE(c->watermark))
> WRITE_ONCE(c->watermark, new);
> }
>
> Cheers,
> Longman
>
Hmm, yeah, given that we'll only be resetting one of the two, I think I'll
use this option.
The branch predictor should make that second check pretty
much a noop in the common-case when we enter the outer if, too.
Thanks!
--
David Finkel
Senior Principal Software Engineer, Core Services
