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From: Pawel Moll <pawel.moll-5wv7dgnIgG8@public.gmane.org>
Subject: Re: [PATCH v4 3/3] perf: Sample additional clock value
Date: Fri, 23 Jan 2015 17:06:07 +0000
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To: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, John Stultz <john.stultz-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
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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