Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

[Avi Kivity <avi@redhat.com>]

On 03/16/2010 09:24 AM, Ingo Molnar wrote:
> * Avi Kivity<avi@redhat.com>  wrote:
>
>    
>> On 03/16/2010 07:27 AM, Zhang, Yanmin wrote:
>>      
>>> From: Zhang, Yanmin<yanmin_zhang@linux.intel.com>
>>>
>>> Based on the discussion in KVM community, I worked out the patch to support
>>> perf to collect guest os statistics from host side. This patch is implemented
>>> with Ingo, Peter and some other guys' kind help. Yang Sheng pointed out a
>>> critical bug and provided good suggestions with other guys. I really appreciate
>>> their kind help.
>>>
>>> The patch adds new subcommand kvm to perf.
>>>
>>>    perf kvm top
>>>    perf kvm record
>>>    perf kvm report
>>>    perf kvm diff
>>>
>>> The new perf could profile guest os kernel except guest os user space, but it
>>> could summarize guest os user space utilization per guest os.
>>>
>>> Below are some examples.
>>> 1) perf kvm top
>>> [root@lkp-ne01 norm]# perf kvm --host --guest --guestkallsyms=/home/ymzhang/guest/kallsyms
>>> --guestmodules=/home/ymzhang/guest/modules top
>>>
>>>        
>> Excellent, support for guest kernel != host kernel is critical (I
>> can't remember the last time I ran same kernels).
>>
>> How would we support multiple guests with different kernels? Perhaps a
>> symbol server that perf can connect to (and that would connect to guests in
>> turn)?
>>      
> The highest quality solution would be if KVM offered a 'guest extension' to
> the guest kernel's /proc/kallsyms that made it easy for user-space to get this
> information from an authorative source.
>
> That's the main reason why the host side /proc/kallsyms is so popular and so
> useful: while in theory it's mostly redundant information which can be gleaned
> from the System.map and other sources of symbol information, it's easily
> available and is _always_ trustable to come from the host kernel.
>
> Separate System.map's have a tendency to go out of sync (or go missing when a
> devel kernel gets rebuilt, or if a devel package is not installed), and server
> ports (be that a TCP port space server or an UDP port space mount-point) are
> both a configuration hassle and are not guest-transparent.
>
> So for instrumentation infrastructure (such as perf) we have a large and well
> founded preference for intrinsic, built-in, kernel-provided information: i.e.
> a largely 'built-in' and transparent mechanism to get to guest symbols.
>    

The symbol server's client can certainly access the bits through vmchannel.

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