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

From: Jiri Olsa
Date: Tue Sep 02 2014 - 05:44:34 EST

On Tue, Sep 02, 2014 at 08:39:56AM +0300, Adrian Hunter wrote:


> >>> -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?
> Imagine that the 'tsc' term did not exist. Intel PT trace data
> would not contain TSC packets, and the decoder would not know how to
> decode them. Then imagine that a new version of the hardware adds
> 'tsc'. It is such a useful feature that we want it by default, but
> older versions of the tools don't know how to decode it, so the
> kernel cannot turn it on by default.
> >
> > 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..
> It is similar to why the kernel does not select mmap2 by default. The
> kernel doesn't know whether the tool supports it.

ook, could you please add some kind of this explanation to the changelog?

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