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

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Message-ID: <e5e92227-0931-dfc1-841e-c036131e66a8@virtuozzo.com>
Date:   Mon, 15 Jan 2018 15:29:21 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] mm/memcg: try harder to decrease
 [memory,memsw].limit_in_bytes



On 01/13/2018 01:57 AM, Shakeel Butt wrote:
> On Fri, Jan 12, 2018 at 4:24 AM, Michal Hocko <mhocko@...nel.org> wrote:
>> On Fri 12-01-18 00:59:38, Andrey Ryabinin wrote:
>>> On 01/11/2018 07:29 PM, Michal Hocko wrote:
>> [...]
>>>> 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.
>>>>
>>>
>>> I did consider this. And I think, I already explained that sort of race in previous email.
>>> Whether "Task B" is really a task in cgroup or it's actually a bunch of reclaimers,
>>> doesn't matter. That doesn't change anything.
>>
>> I would _really_ prefer two patches here. The first one removing the
>> hard coded reclaim count. That thing is just dubious at best. If you
>> _really_ think that the higher reclaim target is meaningfull then make
>> it a separate patch. I am not conviced but I will not nack it it either.
>> But it will make our life much easier if my over reclaim concern is
>> right and we will need to revert it. Conceptually those two changes are
>> independent anywa.
>>
> 
> Personally I feel that the cgroup-v2 semantics are much cleaner for
> setting limit. There is no race with the allocators in the memcg,
> though oom-killer can be triggered. For cgroup-v1, the user does not
> expect OOM killer and EBUSY is expected on unsuccessful reclaim. How
> about we do something similar here and make sure oom killer can not be
> triggered for the given memcg?
> 
> // pseudo code
> disable_oom(memcg)
> old = xchg(&memcg->memory.limit, requested_limit)
> 
> reclaim memory until usage gets below new limit or retries are exhausted
> 
> if (unsuccessful) {
>   reset_limit(memcg, old)
>   ret = EBUSY
> } else
>   ret = 0;
> enable_oom(memcg)
> 
> This way there is no race with the allocators and oom killer will not
> be triggered. The processes in the memcg can suffer but that should be
> within the expectation of the user. One disclaimer though, disabling
> oom for memcg needs more thought.
 
That's might be worse. If limit is too low, all allocations (except __GFP_NOFAIL of course) will start
failing. And the kernel not always careful enough in -ENOMEM handling.
Also, it's not much different from oom killing everything, the end result is almost the same -
nothing will work in that cgroup.


> Shakeel
>