linux-kernel - Re: [v9 PATCH 13/13] mm: vmscan: shrink deferred objects proportional to priority

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Date:   Wed, 10 Mar 2021 10:54:43 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <guro@...com>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Chinner <david@...morbit.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [v9 PATCH 13/13] mm: vmscan: shrink deferred objects proportional
 to priority

On Wed, Mar 10, 2021 at 10:24 AM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Wed, Mar 10, 2021 at 9:46 AM Yang Shi <shy828301@...il.com> wrote:
> >
> > The number of deferred objects might get windup to an absurd number, and it
> > results in clamp of slab objects.  It is undesirable for sustaining workingset.
> >
> > So shrink deferred objects proportional to priority and cap nr_deferred to twice
> > of cache items.
> >
> > The idea is borrowed from Dave Chinner's patch:
> > https://lore.kernel.org/linux-xfs/20191031234618.15403-13-david@fromorbit.com/
> >
> > Tested with kernel build and vfs metadata heavy workload in our production
> > environment, no regression is spotted so far.
>
> Did you run both of these workloads in the same cgroup or separate cgroups?

Both are covered.

>
> >
> > Signed-off-by: Yang Shi <shy828301@...il.com>
> > ---
> >  mm/vmscan.c | 46 +++++++++++-----------------------------------
> >  1 file changed, 11 insertions(+), 35 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9a2dfeaa79f4..6a0a91b23597 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -662,7 +662,6 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> >          */
> >         nr = xchg_nr_deferred(shrinker, shrinkctl);
> >
> > -       total_scan = nr;
> >         if (shrinker->seeks) {
> >                 delta = freeable >> priority;
> >                 delta *= 4;
> > @@ -676,37 +675,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> >                 delta = freeable / 2;
> >         }
> >
> > +       total_scan = nr >> priority;
> >         total_scan += delta;
> > -       if (total_scan < 0) {
> > -               pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n",
> > -                      shrinker->scan_objects, total_scan);
> > -               total_scan = freeable;
> > -               next_deferred = nr;
> > -       } else
> > -               next_deferred = total_scan;
> > -
> > -       /*
> > -        * We need to avoid excessive windup on filesystem shrinkers
> > -        * due to large numbers of GFP_NOFS allocations causing the
> > -        * shrinkers to return -1 all the time. This results in a large
> > -        * nr being built up so when a shrink that can do some work
> > -        * comes along it empties the entire cache due to nr >>>
> > -        * freeable. This is bad for sustaining a working set in
> > -        * memory.
> > -        *
> > -        * Hence only allow the shrinker to scan the entire cache when
> > -        * a large delta change is calculated directly.
> > -        */
> > -       if (delta < freeable / 4)
> > -               total_scan = min(total_scan, freeable / 2);
> > -
> > -       /*
> > -        * Avoid risking looping forever due to too large nr value:
> > -        * never try to free more than twice the estimate number of
> > -        * freeable entries.
> > -        */
> > -       if (total_scan > freeable * 2)
> > -               total_scan = freeable * 2;
> > +       total_scan = min(total_scan, (2 * freeable));
> >
> >         trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
> >                                    freeable, delta, total_scan, priority);
> > @@ -745,10 +716,15 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> >                 cond_resched();
> >         }
> >
> > -       if (next_deferred >= scanned)
> > -               next_deferred -= scanned;
> > -       else
> > -               next_deferred = 0;
> > +       /*
> > +        * The deferred work is increased by any new work (delta) that wasn't
> > +        * done, decreased by old deferred work that was done now.
> > +        *
> > +        * And it is capped to two times of the freeable items.
> > +        */
> > +       next_deferred = max_t(long, (nr + delta - scanned), 0);
> > +       next_deferred = min(next_deferred, (2 * freeable));
> > +
> >         /*
> >          * move the unused scan count back into the shrinker in a
> >          * manner that handles concurrent updates.
> > --
> > 2.26.2
> >