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

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Date:   Tue, 24 Apr 2018 07:55:07 +0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     Wei Liu <wei.liu2@...rix.com>
Cc:     jgross@...e.com, ian.jackson@...citrix.com,
        linux-kernel@...r.kernel.org, srinivas.eeda@...cle.com,
        xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
        Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [Xen-devel] [RFC 0/2] To introduce xenwatch multithreading (xen
 mtwatch)

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.

Dongli Zhang