linux-kernel - Re: [PATCH] mm, vmscan: Don't turn on cache_trim

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Date: Fri, 16 Feb 2024 16:24:34 +0900
From: Byungchul Park <byungchul@...com>
To: Yu Zhao <yuzhao@...gle.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel_team@...ynix.com
Subject: Re: [PATCH] mm, vmscan: Don't turn on cache_trim_mode at the highest
 scan priority

On Fri, Feb 16, 2024 at 12:55:17AM -0500, Yu Zhao wrote:
> On Thu, Feb 8, 2024 at 1:18 AM Byungchul Park <byungchul@...com> wrote:
> >
> > With cache_trim_mode on, reclaim logic doesn't bother reclaiming anon
> > pages. However, it should be more careful to turn on the mode because
> > it's going to prevent anon pages from reclaimed even if there are huge
> > ammount of anon pages that are very cold so should be reclaimed. Even
> > worse, that can lead kswapd_failures to be MAX_RECLAIM_RETRIES and stop
> > until direct reclaim eventually works to resume kswapd.
> 
> Is a theory or something observed in the real world? If it's the
> former, would this change risk breaking existing use cases? It's the

I faced the latter case.

> latter, where are the performance numbers to show what it looks like
> before and after this patch?

Before:

Whenever the system meets the condition to turn on cache_trim_mode but
few cache pages to trim, kswapd fails without scanning anon pages that
are plenty and cold for sure and it retries 8 times and looks *stopped
for ever*.

After:

When the system meets the condition to turn on cache_trim_mode but few
cache pages to trim, kswapd finally works at the highest scan priority.
So kswap looks working well even in the same condition.

> > Signed-off-by: Byungchul Park <byungchul@...com>
> > ---
> >  mm/vmscan.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index bba207f41b14..25b55fdc0d41 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2268,7 +2268,8 @@ static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc)
> >          * anonymous pages.
> >          */
> >         file = lruvec_page_state(target_lruvec, NR_INACTIVE_FILE);
> > -       if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE))
> > +       if (sc->priority != 1 && file >> sc->priority &
> 
> Why 1?

It means the highest scan priority. The priority goes from DEF_PRIORITY
to 1.

	Byungchul

> > +           !(sc->may_deactivate & DEACTIVATE_FILE))
> >                 sc->cache_trim_mode = 1;
> >         else
> >                 sc->cache_trim_mode = 0;