Re: [Xen-devel] [RFC 0/2] To introduce xenwatch multithreading (xen mtwatch)

[Dongli Zhang <dongli.zhang@oracle.com>]

On 04/24/2018 02:03 PM, Juergen Gross wrote:
> On 24/04/18 07:52, Dongli Zhang wrote:
>> Hi Juergen,
>>
>> On 04/24/2018 01:22 PM, Juergen Gross wrote:
>>> On 24/04/18 01:55, Dongli Zhang wrote:
>>>> Hi Wei,
>>>>
>>>> On 04/23/2018 10:09 PM, Wei Liu wrote:
>>>>> On Sat, Apr 07, 2018 at 07:25:53PM +0800, Dongli Zhang wrote:
>>>>>> About per-domU xenwatch thread create/destroy, a new type of xenstore node is
>>>>>> introduced: '/local/domain/0/mtwatch/<domid>'.
>>>>>>
>>>>>> Suppose the new domid id 7. During the domU (domid=7) creation, the xen
>>>>>> toolstack writes '/local/domain/0/mtwatch/7' to xenstore before the insertion
>>>>>> of '/local/domain/7'. When the domid=7 is destroyed, the last xenstore
>>>>>> operation by xen toolstack is to remove '/local/domain/0/mtwatch/7'.
>>>>>>
>>>>>> The dom0 kernel subscribes a watch at node '/local/domain/0/mtwatch'.  Kernel
>>>>>> thread [xen-mtwatch-7] is created when '/local/domain/0/mtwatch/7' is inserted,
>>>>>> while this kernel thread is destroyed when the corresponding xenstore node is
>>>>>> removed.
>>>>>
>>>>> Instead of inventing yet another node, can you not watch /local/domain
>>>>> directly?
>>>>
>>>> Would you like to watch at /local/domain directly? Or is your question "is there
>>>> any other way to not watch at /local/domain, while no extra xenstore node will
>>>> be introduced"?
>>>>
>>>> Actually, the first prototype of this idea was to watch at /local/domain
>>>> directly to get aware of the domU create/destroy, so that xen toolstack will not
>>>> get involved. Joao Martins (CCed) had a concern on the performance as watching
>>>> at /local/domain would lead to large amount of xenwatch events.
>>>
>>> That's what the special watches "@introduceDomain" and "@releaseDomain"
>>> are meant for.
>>
>> I used to consider to watch at "@introduceDomain". However, there is no domain
>> information appended with "@introduceDomain" and it is still required for dom0
>> kernel to proactively confirm who is created.
> 
> That isn't too hard, right? You just need to read /local/domain to get
> the list of its children and look for new domains there.

You are right. I will try to limit the modification within linux kernel, and try
to not dirty xen toolstack.

Thank you very much for the suggestion.

Dongli Zhang