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

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Message-ID: <20180111162947.GG1732@dhcp22.suse.cz>
Date:   Thu, 11 Jan 2018 17:29:47 +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 18:23:57, Andrey Ryabinin wrote:
> On 01/11/2018 03:46 PM, Michal Hocko wrote:
> > 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?
> > 
> 
> What's insane about that?

I haven't seen this being done in practice. Why would you want to
reclaim GBs of memory from a cgroup? Anyway, if you believe this is
really needed then simply do it in a separate patch.
 
> >> @@ -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?
>  
> It protects from over reclaim due to underflow.

I do not think so. Consider that this reclaim races with other
reclaimers. Now you are reclaiming a large chunk so you might end up
reclaiming more than necessary. SWAP_CLUSTER_MAX would reduce the over
reclaim to be negligible.
-- 
Michal Hocko
SUSE Labs