Re: [PATCH v4 3/3] perf: Sample additional clock value

[Pawel Moll <pawel.moll@arm.com>]

On Mon, 2015-01-05 at 13:45 +0000, Peter Zijlstra wrote:
> Also, one would expect something like:
> 
> 	default: {
> 		struct k_clock *kc = clockid_to_kclock(event->attr.clock);
> 		struct timespec ts;
> 		if (kc) {
> 			kc->clock_get(event->attr.clock, &ts);
> 			data->clock = ktime_to_ns(timespec_to_ktime(ts));
> 		} else {
> 			data->clock = 0;
> 		}
> 	}
> 
> Albeit preferably slightly less horrible -- of course, one would first
> need to deal with the NMI issue.

I was thinking about it... Maybe the solution is approaching the problem
in a completely different way.

As far as I understand (John?) POSIX timers can be used on any clockid?
So it would be possible to obtain a dynamic clock id, for example for my
exotic trace hardware (by any means necessary, like opening a char
device) and create a timer firing every 1 ms (in the trace time domain).
Than this event would be somehow associated with a perf session (for
example, by passing the timerid via perf's ioctl) and then, every when
timer fires, a perf record (something like PERF_RECORD_TIMER?)
containing the timer/clock's value *and* the normal perf timestamp,
would be injected into the circular buffer.

No issue with NMI, no issue with passing clockid through
perf_event_attr...

Does it make any sense to anyone else but me? ;-)

Pawel