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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n>
 caches
To:     Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>
Cc:     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
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Message-ID: <95ab4bdf-f373-f71d-cdf5-05ad6edbb772@redhat.com>
Date:   Wed, 5 May 2021 19:06:24 -0400
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On 5/5/21 6:19 PM, Roman Gushchin wrote:
> 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).
>
The criteria for cache merging require close to exact match in all 
attributes with a size difference of no more than sizeof(void *). So it 
is not easy to find a close match.

Cheers,
Longman