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

[David Ahern <daahern@cisco.com>]

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.

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.

David


> 
>> @@ -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.

David


> 
> 
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