Re: [RFC] [PATCH 1/1] perf: add support for arch-dependent symbolicevent names to "perf stat"

From: Corey Ashford
Date: Thu Mar 04 2010 - 13:39:33 EST

On 3/3/2010 6:30 PM, Corey Ashford wrote:
For your review, this patch adds support for arch-dependent symbolic
event names to the "perf stat" tool, and could be expanded to other
"perf *" commands fairly easily, I suspect.

To support arch-dependent event names without adding arch-dependent code
to perf, I added a callout mechanism whereby perf will look for the
environment variable: PERF_ARCH_DEP_LIB, and if it exists, it will try
to open it as a shared object. If that succeeds, it looks for the symbol
"parse_arch_dep_event". If that exists, that function will be called by
parse_events() before all of the other event parsing functions in
parse-events.c. It is passed the same arguments as the other
parse_*_event functions, namely the event string and a pointer to an
event attribute structure.

As the code existed, "perf stat" would print out the count results, but
for raw events (which is how arch-dependent events are supported in
perf_events), it would just print out a raw code. This is not
acceptable, especially when a symbolic name was placed on the command
line. So I changed the code to save away the event name that was passed
on the command line, rather than doing a reverse translation to an event
string based on the event type and config fields of the attr structure.
In this way, there's no need for a reverse translation function in the
arch-dependent library; only a event string->attr struct function is

I could well be missing something, but I don't understand why reverse
translation is ever needed in perf, as long as the tool keeps track of
the original event strings.

A couple of follow-up comments on this patch:

This functionality was designed to provide a generalized interface to an external event name -> attr struct library, such as libpfm4. libpfm4 has an interface that nearly exactly matches parse_*_event() profiles, so it's quite easy to write a small wrapper function to call libpfm4's function.

Ingo Molnar discussed adding some visibility to the arch-dependent event names through some other interface, such as through /sys/devices/pmus perhaps, but that discussion is a long way (as far as I know) from having something usable today. So you could think of this external library approach to be a stop-gap until something better is developed. When/if that new event naming mechanism becomes available, we can easily remove this external library support from perf.


- Corey

Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR

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