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

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Date:	Fri, 23 Jan 2015 17:06:07 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christopher Covington <cov@...eaurora.org>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] perf: Sample additional clock value

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

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