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

From: Juergen Gross
Date: Tue Apr 24 2018 - 02:23:47 EST

On 24/04/18 08:10, Dongli Zhang wrote:
> 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.

When going that route you should extend xenbus_directory() in the kernel
to use XS_DIRECTORY_PART in case XS_DIRECTORY returns E2BIG (see
xs_directory() in Xen's tools/xenstore/xs.c ). This will enable the
kernel to support more than about 1000 domains.