linux-kernel - Re: [PATCH 1/1] perf/x86/intel: make error messages less confusing

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Date:   Wed, 22 Aug 2018 13:57:07 -0700
From:   Eduardo Valentin <eduval@...zon.com>
To:     Andi Kleen <ak@...ux.intel.com>
CC:     Eduardo Valentin <eduval@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Kan Liang" <kan.liang@...ux.intel.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Jia Zhang <qianyue.zj@...baba-inc.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] perf/x86/intel: make error messages less confusing

Hey,

On Tue, Aug 21, 2018 at 04:59:08PM -0700, Andi Kleen wrote:
> On Tue, Aug 21, 2018 at 04:05:22PM -0700, Eduardo Valentin wrote:
> > On Tue, Aug 21, 2018 at 03:09:37PM -0700, Andi Kleen wrote:
> > > On Tue, Aug 21, 2018 at 02:15:28PM -0700, Eduardo Valentin wrote:
> > > > On a system with X86_FEATURE_ARCH_PERFMON disabled
> > > > and with a model not known by family PMU drivers,
> > > > user gets a kernel message log like the following:
> > > > [ 0.100114] Performance Events: unsupported p6 CPU model 85 no PMU driver, software events only.
> > > > 
> > > > The "unsupported .. CPU" part may be confusing for some
> > > > users. Rewording the messages on the failure path to:
> > > > [ 0.667154] Performance Events: unknown p6 PMU on CPU model 85: !X86_FEATURE_ARCH_PERFMON: no PMU driver, software events only.
> > > 
> > > Are you sure users even know what ARCH_PERFMON is?
> > > 
> > > Maybe it is confusing (why exactly?), but it doesn't seem to me that your
> > > new message is any better.
> > 
> > Yeah, the part that says "unsupported CPU" is the confusing part,
> 
> That makes sense.
> 
> > I get people thinking that the specific reported CPU model is not
> > supported by the kernel :-)
> > 
> > > 
> > > If you refer to VMs not exposing the PMU perhaps that should be explicitely mentioned.
> > > 
> > > Of course the real fix is to always expose the PMU, not improve the error messages...
> > 
> > I agree that best is simply to enable PMU. But it does not hurt to improve the error messaging, does it?
> > 
> > Any suggestions there, given that the initial attempt seams to make it even worse :-)
> 
> Perhaps just say
> 
> "CPU does not support PMU"
>

Ok.
 
> which is really what the problem is here.
> 

Fair enough.

> The other option would be to move this message after the big model switch,
> but would need to be very careful that it doesn't have any unintended
> side effects. 

I will simply remove those messages with CPU model details and on
the failure path build the message to look like:
[    0.666785] Performance Events: CPU does not support PMU: no PMU driver, software events only.


> 
> -Andi
> 

-- 
All the best,
Eduardo Valentin