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Subject: Re: [Xen-devel] [RFC 0/2] To introduce xenwatch multithreading (xen
 mtwatch)
To:     Dongli Zhang <dongli.zhang@oracle.com>,
        Wei Liu <wei.liu2@citrix.com>
Cc:     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>
References: <1523100355-27647-1-git-send-email-dongli.zhang@oracle.com>
 <20180423140911.bihiwfwtlneobgzd@citrix.com>
 <6c379249-886d-76ee-db9c-942967b23d80@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <db08fec6-5648-4d87-4ace-2f42d1d04479@suse.com>
Date:   Tue, 24 Apr 2018 07:22:56 +0200
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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.


Juergen