Re: [PATCH v2] mm: slowly shrink slabs with a relatively small number of objects

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 4 Sep 2018 15:47:07 -0700 Roman Gushchin <guro@fb.com> wrote:

> Commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
> changed the way how the target slab pressure is calculated and
> made it priority-based:
> 
>     delta = freeable >> priority;
>     delta *= 4;
>     do_div(delta, shrinker->seeks);
> 
> The problem is that on a default priority (which is 12) no pressure
> is applied at all, if the number of potentially reclaimable objects
> is less than 4096 (1<<12).
> 
> This causes the last objects on slab caches of no longer used cgroups
> to never get reclaimed, resulting in dead cgroups staying around forever.

But this problem pertains to all types of objects, not just the cgroup
cache, yes?

> Slab LRU lists are reparented on memcg offlining, but corresponding
> objects are still holding a reference to the dying cgroup.
> If we don't scan them at all, the dying cgroup can't go away.
> Most likely, the parent cgroup hasn't any directly associated objects,
> only remaining objects from dying children cgroups. So it can easily
> hold a reference to hundreds of dying cgroups.
> 
> If there are no big spikes in memory pressure, and new memory cgroups
> are created and destroyed periodically, this causes the number of
> dying cgroups grow steadily, causing a slow-ish and hard-to-detect
> memory "leak". It's not a real leak, as the memory can be eventually
> reclaimed, but it could not happen in a real life at all. I've seen
> hosts with a steadily climbing number of dying cgroups, which doesn't
> show any signs of a decline in months, despite the host is loaded
> with a production workload.
> 
> It is an obvious waste of memory, and to prevent it, let's apply
> a minimal pressure even on small shrinker lists. E.g. if there are
> freeable objects, let's scan at least min(freeable, scan_batch)
> objects.
> 
> This fix significantly improves a chance of a dying cgroup to be
> reclaimed, and together with some previous patches stops the steady
> growth of the dying cgroups number on some of our hosts.
> 
> ...
>
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -476,6 +476,17 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	delta = freeable >> priority;
>  	delta *= 4;
>  	do_div(delta, shrinker->seeks);
> +
> +	/*
> +	 * Make sure we apply some minimal pressure even on
> +	 * small cgroups. This is necessary because some of
> +	 * belonging objects can hold a reference to a dying
> +	 * child cgroup. If we don't scan them, the dying
> +	 * cgroup can't go away unless the memory pressure
> +	 * (and the scanning priority) raise significantly.
> +	 */
> +	delta = max(delta, min(freeable, batch_size));
> +

If so I think the comment should be cast in more general terms.  Maybe
with a final sentence "the cgroup cache is one such case".

Also, please use all 80 columns in block comments to save a few display
lines.

And `delta' has type ULL whereas the other two are longs.  We'll
presumably hit warnings here, preventable with max_t.