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

    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
    >> <>.
    >> (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.

    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.


    >> @@ -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);
    >> + = &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.


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