[dm-devel] [PATCH] dm bufio: fix shrinker scans when (nr_to_scan < retain_target)

[Suren Baghdasaryan]

Dear kernel maintainers. I know it was close to holiday season when I
send this patch last month, so delay was expected. Could you please
take a look at it and provide your feedback?
Thanks!

On Wed, Dec 6, 2017 at 9:27 AM, Suren Baghdasaryan <surenb at google.com> wrote:
> When system is under memory pressure it is observed that dm bufio
> shrinker often reclaims only one buffer per scan. This change fixes
> the following two issues in dm bufio shrinker that cause this behavior:
>
> 1. ((nr_to_scan - freed) <= retain_target) condition is used to
> terminate slab scan process. This assumes that nr_to_scan is equal
> to the LRU size, which might not be correct because do_shrink_slab()
> in vmscan.c calculates nr_to_scan using multiple inputs.
> As a result when nr_to_scan is less than retain_target (64) the scan
> will terminate after the first iteration, effectively reclaiming one
> buffer per scan and making scans very inefficient. This hurts vmscan
> performance especially because mutex is acquired/released every time
> dm_bufio_shrink_scan() is called.
> New implementation uses ((LRU size - freed) <= retain_target)
> condition for scan termination. LRU size can be safely determined
> inside __scan() because this function is called after dm_bufio_lock().
>
> 2. do_shrink_slab() uses value returned by dm_bufio_shrink_count() to
> determine number of freeable objects in the slab. However dm_bufio
> always retains retain_target buffers in its LRU and will terminate
> a scan when this mark is reached. Therefore returning the entire LRU size
> from dm_bufio_shrink_count() is misleading because that does not
> represent the number of freeable objects that slab will reclaim during
> a scan. Returning (LRU size - retain_target) better represents the
> number of freeable objects in the slab. This way do_shrink_slab()
> returns 0 when (LRU size < retain_target) and vmscan will not try to
> scan this shrinker avoiding scans that will not reclaim any memory.
>
> Test: tested using Android device running
> <AOSP>/system/extras/alloc-stress that generates memory pressure
> and causes intensive shrinker scans
>
> Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> ---
>  drivers/md/dm-bufio.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> index b8ac591aaaa7..c546b567f3b5 100644
> --- a/drivers/md/dm-bufio.c
> +++ b/drivers/md/dm-bufio.c
> @@ -1611,7 +1611,8 @@ static unsigned long __scan(struct dm_bufio_client *c, unsigned long nr_to_scan,
>         int l;
>         struct dm_buffer *b, *tmp;
>         unsigned long freed = 0;
> -       unsigned long count = nr_to_scan;
> +       unsigned long count = c->n_buffers[LIST_CLEAN] +
> +                             c->n_buffers[LIST_DIRTY];
>         unsigned long retain_target = get_retain_buffers(c);
>
>         for (l = 0; l < LIST_SIZE; l++) {
> @@ -1647,8 +1648,11 @@ static unsigned long
>  dm_bufio_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
>  {
>         struct dm_bufio_client *c = container_of(shrink, struct dm_bufio_client, shrinker);
> +       unsigned long count = READ_ONCE(c->n_buffers[LIST_CLEAN]) +
> +                             READ_ONCE(c->n_buffers[LIST_DIRTY]);
> +       unsigned long retain_target = get_retain_buffers(c);
>
> -       return READ_ONCE(c->n_buffers[LIST_CLEAN]) + READ_ONCE(c->n_buffers[LIST_DIRTY]);
> +       return (count < retain_target) ? 0 : (count - retain_target);
>  }
>
>  /*
> --
> 2.15.0.531.g2ccb3012c9-goog
>