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Subject: Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty
To:     Greg Thelen <gthelen@google.com>, Michal Hocko <mhocko@kernel.org>
Cc:     hannes@cmpxchg.org, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <5d9579aa-cfdc-8254-bfd9-63c4e1bfa4c5@linux.alibaba.com>
Date:   Fri, 4 Jan 2019 14:57:33 -0800
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On 1/4/19 12:03 PM, Greg Thelen wrote:
> Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>> On 1/3/19 11:23 AM, Michal Hocko wrote:
>>> On Thu 03-01-19 11:10:00, Yang Shi wrote:
>>>> On 1/3/19 10:53 AM, Michal Hocko wrote:
>>>>> On Thu 03-01-19 10:40:54, Yang Shi wrote:
>>>>>> On 1/3/19 10:13 AM, Michal Hocko wrote:
>>> [...]
>>>>>>> Is there any reason for your scripts to be strictly sequential here? In
>>>>>>> other words why cannot you offload those expensive operations to a
>>>>>>> detached context in _userspace_?
>>>>>> I would say it has not to be strictly sequential. The above script is just
>>>>>> an example to illustrate the pattern. But, sometimes it may hit such pattern
>>>>>> due to the complicated cluster scheduling and container scheduling in the
>>>>>> production environment, for example the creation process might be scheduled
>>>>>> to the same CPU which is doing force_empty. I have to say I don't know too
>>>>>> much about the internals of the container scheduling.
>>>>> In that case I do not see a strong reason to implement the offloding
>>>>> into the kernel. It is an additional code and semantic to maintain.
>>>> Yes, it does introduce some additional code and semantic, but IMHO, it is
>>>> quite simple and very straight forward, isn't it? Just utilize the existing
>>>> css offline worker. And, that a couple of lines of code do improve some
>>>> throughput issues for some real usecases.
>>> I do not really care it is few LOC. It is more important that it is
>>> conflating force_empty into offlining logic. There was a good reason to
>>> remove reparenting/emptying the memcg during the offline. Considering
>>> that you can offload force_empty from userspace trivially then I do not
>>> see any reason to implement it in the kernel.
>> Er, I may not articulate in the earlier email, force_empty can not be
>> offloaded from userspace *trivially*. IOWs the container scheduler may
>> unexpectedly overcommit something due to the stall of synchronous force
>> empty, which can't be figured out by userspace before it actually
>> happens. The scheduler doesn't know how long force_empty would take. If
>> the force_empty could be offloaded by kernel, it would make scheduler's
>> life much easier. This is not something userspace could do.
> If kernel workqueues are doing more work (i.e. force_empty processing),
> then it seem like the time to offline could grow.  I'm not sure if
> that's important.

One thing I can think of is this may slow down the recycling of memcg 
id. This may cause memcg id exhausted for some extreme workload. But, I 
don't see this as a problem in our workload.

Thanks,
Yang

>
> I assume that if we make force_empty an async side effect of rmdir then
> user space scheduler would not be unable to immediately assume the
> rmdir'd container memory is available without subjecting a new container
> to direct reclaim.  So it seems like user space would use a mechanism to
> wait for reclaim: either the existing sync force_empty or polling
> meminfo/etc waiting for free memory to appear.
>
>>>>> I think it is more important to discuss whether we want to introduce
>>>>> force_empty in cgroup v2.
>>>> We would prefer have it in v2 as well.
>>> Then bring this up in a separate email thread please.
>> Sure. Will prepare the patches later.
>>
>> Thanks,
>> Yang