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

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Date:	Tue, 16 Mar 2010 10:53:36 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Sheng Yang <sheng@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, ziteng.huang@...el.com
Subject: Re: [PATCH] Enhance perf to collect KVM guest os statistics from
 host side


* Avi Kivity <avi@...hat.com> wrote:

> On 03/16/2010 09:24 AM, Ingo Molnar wrote:
> >* Avi Kivity<avi@...hat.com>  wrote:
> >
> >>On 03/16/2010 07:27 AM, Zhang, Yanmin wrote:
> >>>From: Zhang, Yanmin<yanmin_zhang@...ux.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@...-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.

Ok, that would work i suspect.

Would be nice to have the symbol server in tools/perf/ and also make it easy 
to add it to the initrd via a .config switch or so.

That would have basically all of the advantages of being built into the kernel 
(availability, configurability, transparency, hackability), while having all 
the advantages of a user-space approach as well (flexibility, extensibility, 
robustness, ease of maintenance, etc.).

If only we had tools/xorg/ integrated via the initrd that way ;-)

Thanks,

	Ingo
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