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

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Message-ID: <20100316072449.GB11881@elte.hu>
Date:	Tue, 16 Mar 2010 08:24:49 +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 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.

Thanks,

	Ingo
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