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

Peter Zijlstra
2011-02-18T11:12:51Z

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)

> @@ -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.