Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

From: Roman Gushchin <guro@fb.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Waiman Long <longman@redhat.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Shakeel Butt <shakeelb@google.com>,
	<linux-kernel@vger.kernel.org>, <cgroups@vger.kernel.org>,
	<linux-mm@kvack.org>
Subject: Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches
Date: Wed, 5 May 2021 15:19:39 -0700	[thread overview]
Message-ID: <YJMZ+542NVnbWgat@carbon.dhcp.thefacebook.com> (raw)
In-Reply-To: <f73cc1c6-2950-c56d-6a57-8cebb23db65b@suse.cz>

On Wed, May 05, 2021 at 11:29:54PM +0200, Vlastimil Babka wrote:
> On 5/5/21 8:32 PM, Roman Gushchin wrote:
> > On Wed, May 05, 2021 at 08:02:06PM +0200, Vlastimil Babka wrote:
> >> On 5/5/21 7:30 PM, Roman Gushchin wrote:
> >> > On Wed, May 05, 2021 at 11:46:13AM -0400, Waiman Long wrote:
> >> >> 
> >> >> With this change, all the objcg pointer array objects will come from
> >> >> KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So
> >> >> both the recursive kfree() problem and non-freeable slab problem are
> >> >> gone. Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer
> >> >> have mixed accounted and unaccounted objects, this will slightly reduce
> >> >> the number of objcg pointer arrays that need to be allocated and save
> >> >> a bit of memory.
> >> > 
> >> > Unfortunately the positive effect of this change will be likely
> >> > reversed by a lower utilization due to a larger number of caches.
> >> > 
> >> > Btw, I wonder if we also need a change in the slab caches merging procedure?
> >> > KMALLOC_NORMAL caches should not be merged with caches which can potentially
> >> > include accounted objects.
> >> 
> >> Good point. But looks like kmalloc* caches are extempt from all merging in
> >> create_boot_cache() via
> >> 
> >> 	s->refcount = -1;       /* Exempt from merging for now */
> > 
> > Wait, s->refcount is adjusted to 1 in create_kmalloc_cache() after calling
> > into create_boot_cache?
> 
> Hmm I missed that
> 
> Now I wonder why all kmalloc caches on my system have 0 aliases :)
> cat /sys/kernel/slab/kmalloc-*/aliases

Yeah, I noticed it too, it's a good question. And I remember a case from
the past when it wasn't true (kmalloc-32 was shared with something else).