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Subject: Re: [Xen-devel] [RFC 0/2] To introduce xenwatch multithreading (xen
 mtwatch)
To:     Juergen Gross <jgross@suse.com>
References: <1523100355-27647-1-git-send-email-dongli.zhang@oracle.com>
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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>
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <9a91b19a-5556-f1ea-92d5-29f2baa0ccd5@oracle.com>
Date:   Tue, 24 Apr 2018 13:52:48 +0800
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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.

Dongli Zhang