Re: [PATCH 11/28] mm: factor shrinker work calculations

Brian Foster
2019-11-04T15:29:50Z

On Fri, Nov 01, 2019 at 10:46:01AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Start to clean up the shrinker code by factoring out the calculation
> that determines how much work to do. This separates the calculation
> from clamping and other adjustments that are done before the
> shrinker work is run. Document the scan batch size calculation
> better while we are there.
>
> Also convert the calculation for the amount of work to be done to
> use 64 bit logic so we don't have to keep jumping through hoops to
> keep calculations within 32 bits on 32 bit systems.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

I assume the kbuild warning thing will be fixed up...

> mm/vmscan.c | 97 ++++++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 70 insertions(+), 27 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a215d71d9d4b..2d39ec37c04d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -459,13 +459,68 @@ EXPORT_SYMBOL(unregister_shrinker);
>
> #define SHRINK_BATCH 128
>
> +/*
> + * Calculate the number of new objects to scan this time around. Return
> + * the work to be done. If there are freeable objects, return that number in
> + * @freeable_objects.
> + */
> +static int64_t shrink_scan_count(struct shrink_control *shrinkctl,
> + struct shrinker *shrinker, int priority,
> + int64_t *freeable_objects)
> +{
> + int64_t delta;
> + int64_t freeable;
> +
> + freeable = shrinker->count_objects(shrinker, shrinkctl);
> + if (freeable == 0 || freeable == SHRINK_EMPTY)
> + return freeable;
> +
> + if (shrinker->seeks) {
> + /*
> + * shrinker->seeks is a measure of how much IO is required to
> + * reinstantiate the object in memory. The default value is 2
> + * which is typical for a cold inode requiring a directory read
> + * and an inode read to re-instantiate.
> + *
> + * The scan batch size is defined by the shrinker priority, but
> + * to be able to bias the reclaim we increase the default batch
> + * size by 4. Hence we end up with a scan batch multipler that
> + * scales like so:
> + *
> + * ->seeks scan batch multiplier
> + * 1 4.00x
> + * 2 2.00x
> + * 3 1.33x
> + * 4 1.00x
> + * 8 0.50x
> + *
> + * IOWs, the more seeks it takes to pull the item into cache,
> + * the smaller the reclaim scan batch. Hence we put more reclaim
> + * pressure on caches that are fast to repopulate and to keep a
> + * rough balance between caches that have different costs.
> + */
> + delta = freeable >> (priority - 2);

Does anything prevent priority < 2 here?

> + do_div(delta, shrinker->seeks);
> + } else {
> + /*
> + * These objects don't require any IO to create. Trim them
> + * aggressively under memory pressure to keep them from causing
> + * refetches in the IO caches.
> + */
> + delta = freeable / 2;
> + }
> +
> + *freeable_objects = freeable;
> + return delta > 0 ? delta : 0;
> +}
> +
> static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> struct shrinker *shrinker, int priority)
> {
> unsigned long freed = 0;
> - unsigned long long delta;
> long total_scan;
> - long freeable;
> + int64_t freeable_objects = 0;
> + int64_t scan_count;
> long nr;
> long new_nr;
> int nid = shrinkctl->nid;
...
> @@ -487,25 +543,11 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> */
> nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
>
> - total_scan = nr;
> - if (shrinker->seeks) {
> - delta = freeable >> priority;
> - delta *= 4;
> - do_div(delta, shrinker->seeks);
> - } else {
> - /*
> - * These objects don't require any IO to create. Trim
> - * them aggressively under memory pressure to keep
> - * them from causing refetches in the IO caches.
> - */
> - delta = freeable / 2;
> - }
> -
> - total_scan += delta;
> + total_scan = nr + scan_count;
> if (total_scan < 0) {
> pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n",
> shrinker->scan_objects, total_scan);
> - total_scan = freeable;
> + total_scan = scan_count;

Same question as before: why the change in assignment? freeable was the
->count_objects() return value, which is now stored in freeable_objects.

FWIW, the change seems to make sense in that it just factors out the
deferred count, but it's not clear if it's intentional...

Brian

> next_deferred = nr;
> } else
> next_deferred = total_scan;
> @@ -522,19 +564,20 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> * 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);
> + if (scan_count < freeable_objects / 4)
> + total_scan = min_t(long, total_scan, freeable_objects / 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;
> + if (total_scan > freeable_objects * 2)
> + total_scan = freeable_objects * 2;
>
> trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
> - freeable, delta, total_scan, priority);
> + freeable_objects, scan_count,
> + total_scan, priority);
>
> /*
> * If the shrinker can't run (e.g. due to gfp_mask constraints), then
> @@ -559,7 +602,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> * possible.
> */
> while (total_scan >= batch_size ||
> - total_scan >= freeable) {
> + total_scan >= freeable_objects) {
> unsigned long ret;
> unsigned long nr_to_scan = min(batch_size, total_scan);
>
> --
> 2.24.0.rc0
>