[lkml]   [2011]   [Feb]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
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.


> --
> To unsubscribe from this list: send the line "unsubscribe linux-perf-users" in
> the body of a message to
> More majordomo info at

 \ /
  Last update: 2011-02-18 15:41    [W:0.057 / U:0.064 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site