Re: [PATCH 2/2] mm, slab: Extend vm/drop_caches to shrink kmem slabs

From: Roman Gushchin
Date: Thu Jun 27 2019 - 17:25:24 EST

On Thu, Jun 27, 2019 at 04:57:50PM -0400, Waiman Long wrote:
> On 6/26/19 4:19 PM, Roman Gushchin wrote:
> >>
> >> +#ifdef CONFIG_MEMCG_KMEM
> >> +static void kmem_cache_shrink_memcg(struct mem_cgroup *memcg,
> >> + void __maybe_unused *arg)
> >> +{
> >> + struct kmem_cache *s;
> >> +
> >> + if (memcg == root_mem_cgroup)
> >> + return;
> >> + mutex_lock(&slab_mutex);
> >> + list_for_each_entry(s, &memcg->kmem_caches,
> >> + memcg_params.kmem_caches_node) {
> >> + kmem_cache_shrink(s);
> >> + }
> >> + mutex_unlock(&slab_mutex);
> >> + cond_resched();
> >> +}
> > A couple of questions:
> > 1) how about skipping already offlined kmem_caches? They are already shrunk,
> > so you probably won't get much out of them. Or isn't it true?
> I have been thinking about that. This patch is based on the linux tree
> and so don't have an easy to find out if the kmem caches have been
> shrinked. Rebasing this on top of linux-next, I can use the
> SLAB_DEACTIVATED flag as a marker for skipping the shrink.
> With all the latest patches, I am still seeing 121 out of a total of 726
> memcg kmem caches (1/6) that are deactivated caches after system bootup
> one of the test systems. My system is still using cgroup v1 and so the
> number may be different in a v2 setup. The next step is probably to
> figure out why those deactivated caches are still there.

It's not a secret: these kmem_caches are holding objects, which are in use.
It's a drawback of the current slab accounting implementation: every
object holds a whole page and the corresponding kmem_cache. It's optimized
for a large number of objects, which are created and destroyed within
the life of the cgroup (e.g. task_structs), and it works worse for long-living
objects like vfs cache.

Long-term I think we need a different implementation for long-living objects,
so that objects belonging to different memory cgroups can share the same page
and kmem_caches.

It's a fairly big change though.

> > 2) what's your long-term vision here? do you think that we need to shrink
> > kmem_caches periodically, depending on memory pressure? how a user
> > will use this new sysctl?
> Shrinking the kmem caches under extreme memory pressure can be one way
> to free up extra pages, but the effect will probably be temporary.
> > What's the problem you're trying to solve in general?
> At least for the slub allocator, shrinking the caches allow the number
> of active objects reported in slabinfo to be more accurate. In addition,
> this allow to know the real slab memory consumption. I have been working
> on a BZ about continuous memory leaks with a container based workloads.
> The ability to shrink caches allow us to get a more accurate memory
> consumption picture. Another alternative is to turn on slub_debug which
> will then disables all the per-cpu slabs.

I see... I agree with Michal here, that extending drop_caches sysctl isn't
the best idea. Isn't it possible to achieve the same effect using slub sysfs?