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Subject: Re: [Xen-devel] [RFC 0/2] To introduce xenwatch multithreading (xen
 mtwatch)
To:     Dongli Zhang <dongli.zhang@oracle.com>
Cc:     Wei Liu <wei.liu2@citrix.com>, ian.jackson@eu.citrix.com,
        linux-kernel@vger.kernel.org, srinivas.eeda@oracle.com,
        xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com,
        Joao Martins <joao.m.martins@oracle.com>
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 <9a91b19a-5556-f1ea-92d5-29f2baa0ccd5@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <0280ae73-fd29-33a6-e4d5-8c83a8c2a5d7@suse.com>
Date:   Tue, 24 Apr 2018 08:03:18 +0200
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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.


Juergen