Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small slabs

[Roman Gushchin]

On Mon, Jan 28, 2019 at 02:35:35PM -0500, Rik van Riel wrote:
> There are a few issues with the way the number of slab objects to
> scan is calculated in do_shrink_slab.  First, for zero-seek slabs,
> we could leave the last object around forever. That could result
> in pinning a dying cgroup into memory, instead of reclaiming it.
> The fix for that is trivial.
> 
> Secondly, small slabs receive much more pressure, relative to their
> size, than larger slabs, due to "rounding up" the minimum number of
> scanned objects to batch_size.
> 
> We can keep the pressure on all slabs equal relative to their size
> by accumulating the scan pressure on small slabs over time, resulting
> in sometimes scanning an object, instead of always scanning several.
> 
> This results in lower system CPU use, and a lower major fault rate,
> as actively used entries from smaller caches get reclaimed less
> aggressively, and need to be reloaded/recreated less often.
> 
> Fixes: 4b85afbdacd2 ("mm: zero-seek shrinkers")
> Fixes: 172b06c32b94 ("mm: slowly shrink slabs with a relatively small number of objects")
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Chris Mason <clm@xxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: kernel-team@xxxxxx
> Tested-by: Chris Mason <clm@xxxxxx>

Hi, Rik!

There is a couple of formatting issues (see below), but other than that
the patch looks very good to me. Thanks!

Acked-by: Roman Gushchin <guro@xxxxxx>

> ---
>  include/linux/shrinker.h |  1 +
>  mm/vmscan.c              | 16 +++++++++++++---
>  2 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 9443cafd1969..7a9a1a0f935c 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -65,6 +65,7 @@ struct shrinker {
>  
>  	long batch;	/* reclaim batch size, 0 = default */
>  	int seeks;	/* seeks to recreate an obj */
> +	int small_scan;	/* accumulate pressure on slabs with few objects */
>  	unsigned flags;
>  
>  	/* These are for internal use */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a714c4f800e9..0e375bd7a8b6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -488,18 +488,28 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  		 * them aggressively under memory pressure to keep
>  		 * them from causing refetches in the IO caches.
>  		 */
> -		delta = freeable / 2;
> +		delta = (freeable + 1)/ 2;
                                      ^
                                      A space is missing here.
>  	}
>  
>  	/*
>  	 * Make sure we apply some minimal pressure on default priority
> -	 * even on small cgroups. Stale objects are not only consuming memory
> +	 * even on small cgroups, by accumulating pressure across multiple
> +	 * slab shrinker runs. Stale objects are not only consuming memory
>  	 * by themselves, but can also hold a reference to a dying cgroup,
>  	 * preventing it from being reclaimed. A dying cgroup with all
>  	 * corresponding structures like per-cpu stats and kmem caches
>  	 * can be really big, so it may lead to a significant waste of memory.
>  	 */
> -	delta = max_t(unsigned long long, delta, min(freeable, batch_size));
> +	if (!delta) {
> +		shrinker->small_scan += freeable;
> +
> +		delta = shrinker->small_scan >> priority;
> +		shrinker->small_scan -= delta << priority;
> +
> +		delta *= 4;
> +		do_div(delta, shrinker->seeks);
> +

This empty line can be removed, I believe.

> +	}
>  
>  	total_scan += delta;
>  	if (total_scan < 0) {
>