linux-kernel - Re: [PATCH 20/41] perf tools: Let a user specify a PMU event without any config terms

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Date:	Mon, 1 Sep 2014 21:11:07 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 20/41] perf tools: Let a user specify a PMU event without
 any config terms

On Mon, Sep 01, 2014 at 09:27:17AM +0300, Adrian Hunter wrote:
> On 08/30/2014 11:53 AM, Jiri Olsa wrote:
> > On Fri, Aug 29, 2014 at 09:48:51PM +0300, Adrian Hunter wrote:
> >> On 16/07/2014 9:22 p.m., Jiri Olsa wrote:
> >>> On Wed, Jul 16, 2014 at 06:04:44PM +0300, Adrian Hunter wrote:
> >>>> On 16/07/2014 5:25 p.m., Jiri Olsa wrote:
> >>>>> On Mon, Jul 14, 2014 at 01:02:44PM +0300, Adrian Hunter wrote:
> >>>>>> This enables a PMU event to be specified in the form:
> >>>>>>
> >>>>>> 	pmu//
> >>>>>>
> >>>>>> which is effectively the same as:
> >>>>>>
> >>>>>> 	pmu/config=0/
> >>>>>>
> >>>>>> This patch is a precursor to defining
> >>>>>> default config for a PMU.
> >>>>>
> >>>>> I understand the need for default config, but could you please elaborate
> >>>>> why do we want to parse 'pmu//' as an event string string?
> >>>>
> >>>> Currently the parser requires the slashes to identify a PMU event
> >>>> as opposed to a hardware or other kind of event.
> >>>
> >>> right, so why do we want to parse 'pmu//' as an event string? ;-)
> >>
> >> I am not sure what you mean.  Here I am using 'pmu' as a placeholder
> >> for a real PMU name.  So actual event strings are 'intel_bts//' or
> >> 'intel_pt//' or 'intel_pt/tsc=0,noretcomp=1/'
> > 
> > so the consequence of default arguments is that you can
> > specify event just by the pmu name, like:
> >   -e intel_pt//
> > 
> > which means (with default attributes):
> >   -e intel_pt/tsc=1,noretcomp=0/
> > 
> > I guess I wanted to hear more elaboration why is this better
> > than the current way we have by defining an alias, like:
> > 
> >   krava alias: "tsc=1,noretcomp=0"
> > 
> >   -e intel_pt/krava/
> > 
> > which gives the same result
> 
> The default value must be provided by perf tools not the kernel e.g.
> an older version of perf tools will not be aware of new hardware
> features that the kernel may know about.  If the kernel enables
> new features by default then the tool may fail.  Thus the default
> value has to be under the control of the tools not the kernel, so
> a sysfs alias will not work.
> 

I dont follow, could u provide some example?

why the tool needs to understand.. it will just take the alias
and apply/use it? The alias follows the format logic, which tells
how to apply values on perf_event_attr.. and the kernel knows best
what's supported in its version..

jirka
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