linux-kernel - Re: [PATCH v1 2/5] mm: zswap: refactor limit checking from zswap

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Date: Thu, 4 Apr 2024 21:15:59 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/5] mm: zswap: refactor limit checking from zswap_store()

On Thu, Apr 4, 2024 at 7:45 PM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Fri, Apr 05, 2024 at 01:35:44AM +0000, Yosry Ahmed wrote:
> > Refactor limit and acceptance threshold checking outside of
> > zswap_store(). This code will be moved around in a following patch, so
> > it would be cleaner to move a function call around.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> > ---
> >  mm/zswap.c | 32 ++++++++++++++++----------------
> >  1 file changed, 16 insertions(+), 16 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 1cf3ab4b22e64..fba8f3c3596ab 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1391,6 +1391,21 @@ static void zswap_fill_page(void *ptr, unsigned long value)
> >       memset_l(page, value, PAGE_SIZE / sizeof(unsigned long));
> >  }
> >
> > +static bool zswap_check_full(void)
> > +{
> > +     unsigned long cur_pages = zswap_total_pages();
> > +     unsigned long thr = zswap_accept_thr_pages();
>
> I know this looks neater, but it adds an extra division to the very
> common path where the limit hasn't been reached yet. It should really
> stay inside the branch.

I assumed the compiler is smart enough to do the division only when
necessary, but I didn't check tbh.

>
> Another option could be to precalculate the max and the accept
> threshold in absolute pages whenever their respective module param
> changes. That would eliminate both divisions from the hot path.

Yeah, that's better and cleaner. I will do that in the next version.

Thanks!

>
> > +     unsigned long max_pages = zswap_max_pages();
> > +
> > +     if (cur_pages >= max_pages) {
> > +             zswap_pool_limit_hit++;
> > +             zswap_pool_reached_full = true;
> > +     } else if (zswap_pool_reached_full && cur_pages <= thr) {
> > +             zswap_pool_reached_full = false;
> > +     }
> > +     return zswap_pool_reached_full;
> > +}
> > +
> >  bool zswap_store(struct folio *folio)
> >  {
> >       swp_entry_t swp = folio->swap;