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

[Michal Hocko]

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@xxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> ---
>  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