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

[Avi Kivity <avi@redhat.com>]

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@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...).

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.

> 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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