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

[Shakeel Butt <shakeelb@google.com>]

On Wed, Mar 10, 2021 at 9:46 AM Yang Shi <shy828301@gmail.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?

>
> Signed-off-by: Yang Shi <shy828301@gmail.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
>