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

Shakeel Butt
2018-01-19T14:49:29Z

On Fri, Jan 19, 2018 at 5:35 AM, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.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-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>

Reviewed-by: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

>> Cc: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>> Cc: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
>> Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
>> Cc: Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> ---
>> 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