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

Ingo Molnar
2010-03-16T07:25:18Z

* 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.

Thanks,

Ingo