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

[Luis Chamberlain]

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.
> 
> > 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.

So.. this is still a work around?

> 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.

So this is a debugging mechanism?

> Anyway, I think this can be useful to others that is why I posted the patch.

Since this is debug stuff, please add this to /proc/sys/debug/ instead.
That would reflect the intention, and would avoid the concern that folks
in production would use these things.

Since we only have 2 users of /proc/sys/debug/ I am now wondering if
would be best to add a new sysctl debug taint flag. This way bug
reports with these stupid knobs can got to /dev/null inbox for bug
reports.

Masami, /proc/sys/debug/kprobes-optimization is debug. Would you be OK
to add the taint for it too?

Masoud, /proc/sys/debug/exception-trace seems to actually be enabled
by default, and its goal seems to be to enable disabling it. So I
don't think it would make sense to taint there.

So.. maybe we need something /proc/sys/taints/ or
/proc/sys/debug/taints/ so its *very* clear this is by no way ever
expected to be used in production.

May even be good to long term add a symlink for vm/drop_caches there
as well?

  Luis