linux-kernel - Re: [PATCH] perf, record: Add clockid parameter

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Message-ID: <20150327172059.GK23123@twins.programming.kicks-ass.net>
Date:	Fri, 27 Mar 2015 18:20:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <dsahern@...il.com>
Cc:	tglx@...utronix.de, acme@...hat.com, jolsa@...hat.com,
	eranian@...gle.com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, john.stultz@...aro.org,
	hpa@...or.com, akpm@...ux-foundation.org, mingo@...nel.org
Subject: Re: [PATCH] perf, record: Add clockid parameter

On Fri, Mar 27, 2015 at 11:11:01AM -0600, David Ahern wrote:
> >  tools/perf/builtin-record.c | 3 +++
> >  tools/perf/perf.h           | 1 +
> >  tools/perf/util/evsel.c     | 5 +++++
> >  3 files changed, 9 insertions(+)
> 
> missing Documentation/perf-record.txt update

--- a/tools/perf/Documentation/perf-record.txt
+++ b/tools/perf/Documentation/perf-record.txt
@@ -250,6 +250,11 @@ is off by default.
 --running-time::
 Record running and enabled time for read events (:S)
 
+-k::
+--clockid::
+Sets the clock id to use for the various time fields in the perf_event_type
+records. See clock_gettime().
+
 SEE ALSO
 --------
 linkperf:perf-stat[1], linkperf:perf-list[1]


> >+++ b/tools/perf/util/evsel.c
> >@@ -761,6 +761,11 @@ void perf_evsel__config(struct perf_evsel *evsel, struct record_opts *opts)
> >  		attr->disabled = 0;
> >  		attr->enable_on_exec = 0;
> >  	}
> >+
> >+	if (opts->clockid >= 0) {
> >+		attr->use_clockid = 1;
> >+		attr->clockid = opts->clockid;
> >+	}
> >  }
> >
> >  static int perf_evsel__alloc_fd(struct perf_evsel *evsel, int ncpus, int nthreads)
> >
> 
> This is a new feature which means use_clockid on older kernels will fail. So
> need to catch that and throw an error -- perhaps yet another probe function.

How does that work? What do I grep to find an example? I figured if the
kernel didn't support the syscall will fail and we'll terminate
someplace.

> Also, if the intent is to allow clock selection per event should there be an
> event modifier as well (see get_event_modifier())?

Nah, its not generally useful to mix clocks in a single recording. That
gets real confused real quick on when which event happened.

The per event configurability is handy to allow different (concurrent)
records different settings.
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