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Date:   Wed, 9 Jan 2019 09:55:34 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH 2/5] mm: memcontrol: do not try to do swap when force
 empty



On 1/4/19 4:43 PM, Shakeel Butt wrote:
> On Fri, Jan 4, 2019 at 4:21 PM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>> The typical usecase of force empty is to try to reclaim as much as
>> possible memory before offlining a memcg.  Since there should be no
>> attached tasks to offlining memcg, the tasks anonymous pages would have
>> already been freed or uncharged.  Even though anonymous pages get
>> swapped out, but they still get charged to swap space.  So, it sounds
>> pointless to do swap for force empty.
>>
>> I tried to dig into the history of this, it was introduced by
>> commit 8c7c6e34a125 ("memcg: mem+swap controller core"), but there is
>> not any clue about why it was done so at the first place.
>>
>> The below simple test script shows slight file cache reclaim improvement
>> when swap is on.
>>
>> echo 3 > /proc/sys/vm/drop_caches
>> mkdir /sys/fs/cgroup/memory/test
>> echo 30 > /sys/fs/cgroup/memory/test/memory.swappiness
>> echo $$ >/sys/fs/cgroup/memory/test/cgroup.procs
>> cat /proc/meminfo | grep ^Cached|awk -F" " '{print $2}'
>> dd if=/dev/zero of=/mnt/test bs=1M count=1024
>> ping localhost > /dev/null &
>> echo 1 > /sys/fs/cgroup/memory/test/memory.force_empty
>> killall ping
>> echo $$ >/sys/fs/cgroup/memory/cgroup.procs
>> cat /proc/meminfo | grep ^Cached|awk -F" " '{print $2}'
>> rmdir /sys/fs/cgroup/memory/test
>> cat /proc/meminfo | grep ^Cached|awk -F" " '{print $2}'
>>
>> The number of page cache is:
>>                          w/o             w/
>> before force empty    1088792        1088784
>> after force empty     41492          39428
>> reclaimed             1047300        1049356
>>
>> Without doing swap, force empty can reclaim 2MB more memory in 1GB page
>> cache.
>>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
>> ---
>>   mm/memcontrol.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index af7f18b..75208a2 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2895,7 +2895,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
>>                          return -EINTR;
>>
>>                  progress = try_to_free_mem_cgroup_pages(memcg, 1,
>> -                                                       GFP_KERNEL, true);
>> +                                                       GFP_KERNEL, false);
> I think we agreed not to change the behavior of force_empty. You can
> customize 'force_empty on wipe_on_offline' to not swapout.

OK, will keep force_empty intact.

Thanks,
Yang

>
>>                  if (!progress) {
>>                          nr_retries--;
>>                          /* maybe some writeback is necessary */
>> --
>> 1.8.3.1
>>

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