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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Nov 12, 2019 at 11:13 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Tue, Nov 12, 2019 at 10:00 AM Johannes Weiner <hannes@cmpxchg.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@cmpxchg.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);
>
> ?

On second thought that proposal would not be correct since
deactivation is not the same as reclaim... So the way it is now looks
correct.

Reviewed-by: Suren Baghdasaryan <surenb@google.com>