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Re: [Qemu-devel] KVM brokenness due to IO thread changes

From: Marcelo Tosatti
Subject: Re: [Qemu-devel] KVM brokenness due to IO thread changes
Date: Tue, 28 Apr 2009 19:21:19 -0300
User-agent: Mutt/1.5.18 (2008-05-17)

On Tue, Apr 28, 2009 at 03:09:57PM -0500, Anthony Liguori wrote:
>>>> this is a heads-up, maybe someone has some time to look into this over
>>>> the day: I seems like the IO thread changes caused a few regressions to
>>>> the KVM mode.
>>>> When I keep this feature disabled, I see strange hick-ups of the event
>>>> delivery mechanism, and the guest stops once in a while for a second or
>>>> so. Attaching strace makes the whole process terminate early (looks like
>>>> it triggers a race in the signal handling). And when I enable the IO
>>>> thread, I immediately get a deadlock on qemu_global_mutex.
>>> Yes its borked. The iothread should signal the vcpu thread whenever it
>>> wants to grab the mutex lock, because unlike kvm-userspace it does not
>>> drop the global mutex when entering guest mode (VCPU_RUN ioctl).
>>> Anthony will commit patches to fix that soon.
>> Looking forward. It's far more efficient to test my infrastructure
>> changes against the KVM mode.
> N.B. he's seeing issues with the IO thread disabled.


Can you please provide more details on how to reproduce it? You're using

All I see, with ping -c 0.1, is an eventual ~= 25ms in response time:

64 bytes from icmp_seq=80 ttl=64 time=0.008 ms
64 bytes from icmp_seq=81 ttl=64 time=0.008 ms
64 bytes from icmp_seq=82 ttl=64 time=0.008 ms
64 bytes from icmp_seq=83 ttl=64 time=0.023 ms
64 bytes from icmp_seq=84 ttl=64 time=0.008 ms
64 bytes from icmp_seq=85 ttl=64 time=0.008 ms
64 bytes from icmp_seq=86 ttl=64 time=0.009 ms

Which is also seen before the patches. 

Or even better, if you can bisect it...

> I've not seen this myself and my patch doesn't fix that problem.
> Regards,
> Anthony Liguori

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