Re: [PATCH v5 2/2] mm/memcontrol.c: Reduce reclaim retries in mem_cgroup_resize_limit()

[Shakeel Butt <shakeelb@google.com>]

On Fri, Jan 19, 2018 at 5:35 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Fri 19-01-18 16:25:44, Andrey Ryabinin wrote:
>> Currently mem_cgroup_resize_limit() retries to set limit after reclaiming
>> 32 pages. It makes more sense to reclaim needed amount of pages right away.
>>
>> This works noticeably faster, especially if 'usage - limit' big.
>> E.g. bringing down limit from 4G to 50M:
>>
>> Before:
>>  # perf stat echo 50M > memory.limit_in_bytes
>>
>>      Performance counter stats for 'echo 50M':
>>
>>             386.582382      task-clock (msec)         #    0.835 CPUs utilized
>>                  2,502      context-switches          #    0.006 M/sec
>>
>>            0.463244382 seconds time elapsed
>>
>> After:
>>  # perf stat echo 50M > memory.limit_in_bytes
>>
>>      Performance counter stats for 'echo 50M':
>>
>>             169.403906      task-clock (msec)         #    0.849 CPUs utilized
>>                     14      context-switches          #    0.083 K/sec
>>
>>            0.199536900 seconds time elapsed
>
> But I am not going ack this one. As already stated this has a risk
> of over-reclaim if there a lot of charges are freed along with this
> shrinking. This is more of a theoretical concern so I am _not_ going to

If you don't mind, can you explain why over-reclaim is a concern at
all? The only side effect of over reclaim I can think of is the job
might suffer a bit over (more swapins & pageins). Shouldn't this be
within the expectation of the user decreasing the limits?

> nack. If we ever see such a problem then reverting this patch should be
> pretty straghtforward.
>
>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

>> Cc: Shakeel Butt <shakeelb@google.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
>> ---
>>  mm/memcontrol.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 9d987f3e79dc..09bac2df2f12 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2448,6 +2448,7 @@ static DEFINE_MUTEX(memcg_limit_mutex);
>>  static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
>>                                  unsigned long limit, bool memsw)
>>  {
>> +     unsigned long nr_pages;
>>       bool enlarge = false;
>>       int ret;
>>       bool limits_invariant;
>> @@ -2479,8 +2480,9 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
>>               if (!ret)
>>                       break;
>>
>> -             if (!try_to_free_mem_cgroup_pages(memcg, 1,
>> -                                     GFP_KERNEL, !memsw)) {
>> +             nr_pages = max_t(long, 1, page_counter_read(counter) - limit);
>> +             if (!try_to_free_mem_cgroup_pages(memcg, nr_pages,
>> +                                             GFP_KERNEL, !memsw)) {
>>                       ret = -EBUSY;
>>                       break;
>>               }
>> --
>> 2.13.6
>>
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>
> --
> Michal Hocko
> SUSE Labs