linux-kernel - Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in

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Date:   Thu, 11 Jan 2018 13:46:29 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH v4] mm/memcg: try harder to decrease
 [memory,memsw].limit_in_bytes

On Thu 11-01-18 15:21:33, Andrey Ryabinin wrote:
> 
> 
> On 01/11/2018 01:42 PM, Michal Hocko wrote:
> > On Wed 10-01-18 15:43:17, Andrey Ryabinin wrote:
> > [...]
> >> @@ -2506,15 +2480,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> >>  		if (!ret)
> >>  			break;
> >>  
> >> -		try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, !memsw);
> >> -
> >> -		curusage = page_counter_read(counter);
> >> -		/* Usage is reduced ? */
> >> -		if (curusage >= oldusage)
> >> -			retry_count--;
> >> -		else
> >> -			oldusage = curusage;
> >> -	} while (retry_count);
> >> +		usage = page_counter_read(counter);
> >> +		if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> >> +						GFP_KERNEL, !memsw)) {
> > 
> > If the usage drops below limit in the meantime then you get underflow
> > and reclaim the whole memcg. I do not think this is a good idea. This
> > can also lead to over reclaim. Why don't you simply stick with the
> > original SWAP_CLUSTER_MAX (aka 1 for try_to_free_mem_cgroup_pages)?
> > 
> 
> Because, if new limit is gigabytes bellow the current usage, retrying to set
> new limit after reclaiming only 32 pages seems unreasonable.

Who would do insanity like that?

> @@ -2487,8 +2487,8 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
>  		if (!ret)
>  			break;
>  
> -		usage = page_counter_read(counter);
> -		if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> +		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;

How does this address the over reclaim concern?
-- 
Michal Hocko
SUSE Labs