linux-kernel - Re: [PATCH 3/3] mm: vmscan: enforce inactive:active ratio at the reclaim root

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Date:   Tue, 12 Nov 2019 11:13:09 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...e.com>, linux-mm <linux-mm@...ck.org>,
        cgroups mailinglist <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...com
Subject: Re: [PATCH 3/3] mm: vmscan: enforce inactive:active ratio at the
 reclaim root

On Tue, Nov 12, 2019 at 10:00 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Sun, Nov 10, 2019 at 06:15:50PM -0800, Suren Baghdasaryan wrote:
> > On Thu, Nov 7, 2019 at 12:53 PM Johannes Weiner <hannes@...xchg.org> wrote:
> > > @@ -2758,7 +2775,17 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> > >                         total_high_wmark += high_wmark_pages(zone);
> > >                 }
> > >
> > > -               sc->file_is_tiny = file + free <= total_high_wmark;
> > > +               /*
> > > +                * Consider anon: if that's low too, this isn't a
> > > +                * runaway file reclaim problem, but rather just
> > > +                * extreme pressure. Reclaim as per usual then.
> > > +                */
> > > +               anon = node_page_state(pgdat, NR_INACTIVE_ANON);
> > > +
> > > +               sc->file_is_tiny =
> > > +                       file + free <= total_high_wmark &&
> > > +                       !(sc->may_deactivate & DEACTIVATE_ANON) &&
> > > +                       anon >> sc->priority;
> >
> > The name of file_is_tiny flag seems to not correspond with its actual
> > semantics anymore. Maybe rename it into "skip_file"?
>
> I'm not a fan of file_is_tiny, but I also don't like skip_file. IMO
> it's better to have it describe a situation instead of an action, in
> case we later want to take additional action for that situation.
>
> Any other ideas? ;)

All other ideas still yield verbs (like sc->prefer_anon). Maybe then
add some comment at the file_is_tiny declaration that it represents
not only the fact that the file LRU is too small to reclaim but also
that there are easily reclaimable anon pages?

>
> > I'm confused about why !(sc->may_deactivate & DEACTIVATE_ANON) should
> > be a prerequisite for skipping file LRU reclaim. IIUC this means we
> > will skip reclaiming from file LRU only when anonymous page
> > deactivation is not allowed. Could you please add a comment explaining
> > this?
>
> The comment above this check tries to explain it: the definition of
> file being "tiny" is dependent on the availability of anon. It's a
> relative comparison.
>
> If file only has a few pages, and anon is easily reclaimable (does not
> require deactivation to reclaim pages), then file is "tiny" and we
> should go after the more plentiful anon pages.

Your above explanation is much clearer to me than the one in the comment :)

>
> If anon is under duress, too, this preference doesn't make sense and
> we should just reclaim both lists equally, as per usual.
>
> Note that I'm not introducing this constraint, I'm just changing how
> it's implemented. From the patch:
>
> > >         /*
> > >          * If the system is almost out of file pages, force-scan anon.
> > > -        * But only if there are enough inactive anonymous pages on
> > > -        * the LRU. Otherwise, the small LRU gets thrashed.
> > >          */
> > > -       if (sc->file_is_tiny &&
> > > -           !inactive_list_is_low(lruvec, false, sc, false) &&
> > > -           lruvec_lru_size(lruvec, LRU_INACTIVE_ANON,
> > > -                           sc->reclaim_idx) >> sc->priority) {
> > > +       if (sc->file_is_tiny) {
> > >                 scan_balance = SCAN_ANON;
> > >                 goto out;
> > >         }
>
> So it's always been checking whether reclaim would deactivate anon,
> and whether inactive_anon has sufficient pages for this priority.

I didn't realize !inactive_list_is_low(lruvec, false, sc, false) is
effectively the same as !(sc->may_deactivate & DEACTIVATE_ANON) but
after re-reading the patch that makes sense... Except when
force_deactivate==true, in which case shouldn't you consider
NR_ACTIVE_ANON as easily reclaimable too? IOW should it be smth like
this:

anon = node_page_state(pgdat, NR_INACTIVE_ANON) +
(sc->force_deactivate ? node_page_state(pgdat, NR_ACTIVE_ANON) : 0);

?