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Subject: Re: [PATCH v2 1/2] mm, slab: Extend slab/shrink to shrink all memcg
 caches
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>
References: <20190717202413.13237-1-longman@redhat.com>
 <20190717202413.13237-2-longman@redhat.com>
 <20190719062052.GK30461@dhcp22.suse.cz>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <caa120bb-bfcc-45ef-08e1-af40e52b43df@redhat.com>
Date:   Fri, 19 Jul 2019 10:09:06 -0400
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On 7/19/19 2:20 AM, Michal Hocko wrote:
> On Wed 17-07-19 16:24:12, Waiman Long wrote:
>> Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
>> file to shrink the slab by flushing out all the per-cpu slabs and free
>> slabs in partial lists. This can be useful to squeeze out a bit more memory
>> under extreme condition as well as making the active object counts in
>> /proc/slabinfo more accurate.
>>
>> This usually applies only to the root caches, as the SLUB_MEMCG_SYSFS_ON
>> option is usually not enabled and "slub_memcg_sysfs=1" not set. Even
>> if memcg sysfs is turned on, it is too cumbersome and impractical to
>> manage all those per-memcg sysfs files in a real production system.
>>
>> So there is no practical way to shrink memcg caches.  Fix this by
>> enabling a proper write to the shrink sysfs file of the root cache
>> to scan all the available memcg caches and shrink them as well. For a
>> non-root memcg cache (when SLUB_MEMCG_SYSFS_ON or slub_memcg_sysfs is
>> on), only that cache will be shrunk when written.
> I would mention that memcg unawareness was an overlook more than
> anything else. The interface is intended to shrink all pcp data of the
> cache. The fact that we are using per-memcg internal caches is an
> implementation detail.
>
>> On a 2-socket 64-core 256-thread arm64 system with 64k page after
>> a parallel kernel build, the the amount of memory occupied by slabs
>> before shrinking slabs were:
>>
>>  # grep task_struct /proc/slabinfo
>>  task_struct        53137  53192   4288   61    4 : tunables    0    0
>>  0 : slabdata    872    872      0
>>  # grep "^S[lRU]" /proc/meminfo
>>  Slab:            3936832 kB
>>  SReclaimable:     399104 kB
>>  SUnreclaim:      3537728 kB
>>
>> After shrinking slabs:
>>
>>  # grep "^S[lRU]" /proc/meminfo
>>  Slab:            1356288 kB
>>  SReclaimable:     263296 kB
>>  SUnreclaim:      1092992 kB
>>  # grep task_struct /proc/slabinfo
>>  task_struct         2764   6832   4288   61    4 : tunables    0    0
>>  0 : slabdata    112    112      0
> Now that you are touching the documentation I would just add a note that
> shrinking might be expensive and block other slab operations so it
> should be used with some care.
>
Good point. I will update the patch to include such a note in the
documentation.

Thanks,
Longman