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

[Sheng Yang <sheng@linux.intel.com>]

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@linux.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...). The other concern is, perf alike mechanism would 
bring a lot more overhead compared to 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.

-- 
regards
Yang, Sheng