Re: [PATCH 2/3] perf events: Introduce realtime clock event

Peter Zijlstra
2011-02-18T14:57:13Z

On Fri, 2011-02-18 at 07:39 -0700, David Ahern wrote:
>
> On 02/18/11 04:14, Peter Zijlstra wrote:
> > On Thu, 2011-02-17 at 22:53 -0700, David Ahern wrote:
> >> The motivation for this event is to convert perf_clock() time stamps
> >> to wall-clock (gettimeofday()) equivalents, including adjustments made
> >> by NTP (e.g., for comparing perf events to other log files).
> >
> >> This patch is based on the monotonic patch by Arnaldo Carvalho de Melo
> >> <acme@redhat.com>.
> >>
> >> (NOTE: Comments from the last review of the timehist patch series
> >> suggested calling this a monotonic clock. I am not trying to be
> >> dense here; since gettimeofday maps to realtime clock I think that
> >> is the better name for it.)
> >
> > Well, the idea was to use CLOCK_MONOTONIC, not to call CLOCK_REALTIME
> > monotonic.
> >
> > I'm really not sure why you want CLOCK_REALTIME and I think
> > CLOCK_MONOTONIC is more useful (I'd argue you want your system logs to
> > contain both, every admin who's ever had to untangle what happened
> > during DST switches will agree)
>
> I believe CLOCK_MONOTONIC is what perf_clock is tied to -- the
> timestamps for PERF_SAMPLE_TIME -- so we already have that.

Its not (it mere _can_ be), it could be tied to the TSC which can
significantly drift wrt CLOCK_MONOTONIC.

> Programs that generate time-of-day output are using gettimeofday which
> is tied to CLOCK_REALTIME. We want to be able to correlate a perf sample
> to an entry in an applications log file.

Well, you can argue those programs are broken :-), Imagine the joys of
trying to figure out wth happens when DST jumps the clock back an hour
and you have an hour of duplicate data.

> >> @@ -5610,6 +5612,13 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer)
> >>
> >> perf_sample_data_init(&data, 0);
> >> data.period = event->hw.last_period;
> >> + if (event->attr.sample_type & PERF_SAMPLE_RAW)
> >> + {
> >> + raw.size = sizeof(u64);
> >> + raw.data = &event->count;
> >> + data.raw = &raw;
> >> + }
> >> +
> >> regs = get_irq_regs();
> >>
> >> if (regs && !perf_exclude_event(event, regs)) {
> >
> >
> > Why!? you already keep ->count = ktime_get_real(), so simply reading the
> > count value will get you the timestamp.. this is superfluous at best.
>
> And that is a conundrum I was stuck on for a while. perf record does not
> sample counters; it only creates sample events. I looked at having perf
> record sample the clock event, but then I would have to synthesize an
> event for the output file. Similarly perf record for hardware counters
> does not show the value of the counter.

Well, but you can fix that, simply add PERF_SAMPLE_READ, no need to
abuse the raw crap.