linux-kernel - Re: [PATCH 1/3] perf/x86: add sysfs entry to freeze counter on SMI

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Date:   Fri, 24 Mar 2017 09:16:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     kan.liang@...el.com, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        eranian@...gle.com, jolsa@...nel.org
Subject: Re: [PATCH 1/3] perf/x86: add sysfs entry to freeze counter on SMI

On Thu, Mar 23, 2017 at 03:23:03PM -0700, Andi Kleen wrote:
> On Thu, Mar 23, 2017 at 09:31:38PM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 23, 2017 at 11:25:49AM -0700, kan.liang@...el.com wrote:
> > > From: Kan Liang <Kan.liang@...el.com>
> > > 
> > > When setting FREEZE_WHILE_SMM bit in IA32_DEBUGCTL, all performance
> > > counters will be effected. There is no way to do per-counter freeze
> > > on smi. So it should not use the per-event interface (e.g. ioctl or
> > > event attribute) to set FREEZE_WHILE_SMM bit.
> > > 
> > > Adds sysfs entry /sys/device/cpu/freeze_on_smi to set FREEZE_WHILE_SMM
> > > bit in IA32_DEBUGCTL. When set, freezes perfmon and trace messages
> > > while in SMM.
> > > Value has to be 0 or 1. It will be applied to all possible cpus.
> > 
> > So is there ever a good reason to not set this?
> 
> That means SMIs become invisible to most performance counters.
> 
> I don't think that's a good default. If the SMI takes 1% of my 
> cycles I want to see it.
> 
> The masking trick is mainly useful when doing --smi-cost

Changelog should spell this out though. It adds a knob, so it should say
why it needs be a knob.