linux-kernel - Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

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Date:	Sun, 15 Mar 2009 11:03:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Kevin Shanahan <kmshanah@...b.org.au>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)


* Avi Kivity <avi@...hat.com> wrote:

> Ingo Molnar wrote:
>>> I've looked at the traces but lack the skill to make any sense out of 
>>> them.
>>>     
>>
>> Do you have specific questions about them that we could answer?
>>   
>
> A general question: what's going on?  I guess this will only 
> be answered by me getting my hands dirty and understanding how 
> ftrace works and how the output maps to what's happening.  
> I'll look at the docs for a while.
>
> A specific question for now is how can I identify long latency 
> within qemu here?  As far as I can tell all qemu latencies in 
> trace6.txt are sub 100ms, which, while long, don't explain the 
> guest stalling for many seconds.

Exactly - that in turn means that there's no scheduler latency 
on the host/native kernel side - in turn it must be a KVM 
related latency. (If there was any host side scheduler wakeup or 
other type of latency we'd see it in the trace.)

The most useful trace would be a specific set of trace_printk() 
calls (available on the latest tracing tree), coupled with a 
hyper_trace_printk() which injects a trace entry from the guest 
side into the host kernel trace buffer. (== that would mean a 
hypercall that does a trace_printk().)

	Ingo
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