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

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