Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

[Yang Shi <yang.shi@linux.alibaba.com>]

On 1/4/19 2:57 PM, Yang Shi wrote:
>
>
> 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.

Actually, sync force_empty should have the same side effect.

Yang

>
> 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
>