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Subject: Re: [PATCH] mm, slab: Extend slab/shrink to shrink all the memcg
 caches
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
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        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>
References: <20190702183730.14461-1-longman@redhat.com>
 <20190702130318.39d187dc27dbdd9267788165@linux-foundation.org>
 <78879b79-1b8f-cdfd-d4fa-610afe5e5d48@redhat.com>
 <20190702143340.715f771192721f60de1699d7@linux-foundation.org>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <c29ff725-95ba-db4d-944f-d33f5f766cd3@redhat.com>
Date:   Wed, 3 Jul 2019 11:21:16 -0400
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On 7/2/19 5:33 PM, Andrew Morton wrote:
> On Tue, 2 Jul 2019 16:44:24 -0400 Waiman Long <longman@redhat.com> wrote:
>
>> On 7/2/19 4:03 PM, Andrew Morton wrote:
>>> On Tue,  2 Jul 2019 14:37:30 -0400 Waiman Long <longman@redhat.com> wrote:
>>>
>>>> Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
>>>> file to shrink the slab by flushing all the per-cpu slabs and free
>>>> slabs in partial lists. This applies only to the root caches, though.
>>>>
>>>> Extends this capability by shrinking all the child memcg caches and
>>>> the root cache when a value of '2' is written to the shrink sysfs file.
>>> Why?
>>>
>>> Please fully describe the value of the proposed feature to or users. 
>>> Always.
>> Sure. Essentially, the sysfs shrink interface is not complete. It allows
>> the root cache to be shrunk, but not any of the memcg caches. 
> But that doesn't describe anything of value.  Who wants to use this,
> and why?  How will it be used?  What are the use-cases?
>
For me, the primary motivation of posting this patch is to have a way to
make the number of active objects reported in /proc/slabinfo more
accurately reflect the number of objects that are actually being used by
the kernel. When measuring changes in slab objects consumption between
successive run of a certain workload, I can more easily see the amount
of increase. Without that, the data will have much more noise and it
will be harder to see a pattern.

Cheers,
Longman