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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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        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        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>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <78879b79-1b8f-cdfd-d4fa-610afe5e5d48@redhat.com>
Date:   Tue, 2 Jul 2019 16:44:24 -0400
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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. 

The same can also be said for others slab sysfs files which show current
cache status. I don't think sysfs files are created for the memcg
caches, but I may be wrong. In many cases, information can be available
elsewhere like the slabinfo file. The shrink operation, however, has no
other alternative available.

>> ...
>>
>> --- a/Documentation/ABI/testing/sysfs-kernel-slab
>> +++ b/Documentation/ABI/testing/sysfs-kernel-slab
>> @@ -429,10 +429,12 @@ KernelVersion:	2.6.22
>>  Contact:	Pekka Enberg <penberg@cs.helsinki.fi>,
>>  		Christoph Lameter <cl@linux-foundation.org>
>>  Description:
>> -		The shrink file is written when memory should be reclaimed from
>> -		a cache.  Empty partial slabs are freed and the partial list is
>> -		sorted so the slabs with the fewest available objects are used
>> -		first.
>> +		A value of '1' is written to the shrink file when memory should
>> +		be reclaimed from a cache.  Empty partial slabs are freed and
>> +		the partial list is sorted so the slabs with the fewest
>> +		available objects are used first.  When a value of '2' is
>> +		written, all the corresponding child memory cgroup caches
>> +		should be shrunk as well.  All other values are invalid.
> One would expect this to be a bitfield, like /proc/sys/vm/drop_caches. 
> So writing 3 does both forms of shrinking.
>
> Yes, it happens to be the case that 2 is a superset of 1, but what
> about if we add "4"?
>
Yes, I can make it into a bit fields of 2 bits, just like
/proc/sys/vm/drop_caches.

Cheers,
Longman