linux-kernel - Re: [PATCH] x86/kvm: Show guest system/user cputime in cpustat

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Date:	Thu, 11 Mar 2010 17:17:26 +0800
From:	Sheng Yang <sheng@...ux.intel.com>
To:	Avi Kivity <avi@...hat.com>, Qing He <qing.he@...el.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] x86/kvm: Show guest system/user cputime in cpustat

On Thursday 11 March 2010 15:50:54 Avi Kivity wrote:
> On 03/11/2010 09:46 AM, Sheng Yang wrote:
> > On Thursday 11 March 2010 15:36:01 Avi Kivity wrote:
> >> On 03/11/2010 09:20 AM, Sheng Yang wrote:
> >>> Currently we can only get the cpu_stat of whole guest as one. This
> >>> patch enhanced cpu_stat with more detail, has guest_system and
> >>> guest_user cpu time statistics with a little overhead.
> >>>
> >>> Signed-off-by: Sheng Yang<sheng@...ux.intel.com>
> >>> ---
> >>>
> >>> This draft patch based on KVM upstream to show the idea. I would split
> >>> it into more kernel friendly version later.
> >>>
> >>> The overhead is, the cost of get_cpl() after each exit from guest.
> >>
> >> This can be very expensive in the nested virtualization case, so I
> >> wouldn't like this to be in normal paths.  I think detailed profiling
> >> like that can be left to 'perf kvm', which only has overhead if enabled
> >> at runtime.
> >
> > Yes, that's my concern too(though nested vmcs/vmcb read already too
> > expensive, they should be optimized...).
> 
> Any ideas on how to do that?  Perhaps use paravirt_ops to covert the
> vmread into a memory read?  We store the vmwrites in the vmcs anyway.

When Qing(CCed) was working on nested VMX in the past, he found PV 
vmread/vmwrite indeed works well(it would write to the virtual vmcs so vmwrite 
can also benefit). Though compared to old machine(one our internal patch shows 
improve more than 5%), NHM get less benefit due to the reduced vmexit cost.

-- 
regards
Yang, Sheng

> 
> > The other concern is, perf alike mechanism would
> > bring a lot more overhead compared to this.
> 
> Ordinarily users won't care if time is spent in guest kernel mode or
> guest user mode.  They want to see which guest is imposing a load on a
> system.  I consider a user profiling a guest from the host an advanced
> and rarer use case, so it's okay to require tools and additional
> overhead for this.
> 
> >> For example you can put the code to note the cpl in a tracepoint which
> >> is enabled dynamically.
> >
> > Yanmin have already implement "perf kvm" to support this. We are just
> > arguing if a normal top-alike mechanism is necessary.
> >
> > I am also considering to make it a feature that can be disabled. But
> > seems it make things complicate and result in uncertain cpustat output.
> 
> I'm not even sure that guest time was a good idea.
> 
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