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

Michal Hocko
2018-01-19T13:35:10Z

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
nack. If we ever see such a problem then reverting this patch should be
pretty straghtforward.

> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.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