linux-kernel - Re: [PATCH 4/8] perf data: Add a 'perf' prefix to the generic fields

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Date:	Thu, 11 Dec 2014 21:22:59 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...nel.org>,
	linux-kernel@...r.kernel.org
CC:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jeremie Galarneau <jgalar@...icios.com>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH 4/8] perf data: Add a 'perf' prefix to the generic fields

On 12/11/2014 08:47 PM, David Ahern wrote:
> On 12/11/14 11:57 AM, Sebastian Andrzej Siewior wrote:
>>>>     $ babeltrace ./ctf-data/
>>>>     [03:19:13.962131936] (+0.000001935) cycles: { }, { perf_ip =
>>>> 0xFFFFFFFF8105443A, perf_tid = 20714, perf_pid = 20714, perf_period
>>>> = 8 }
>>>>     [03:19:13.962133732] (+0.000001796) cycles: { }, { perf_ip =
>>>> 0xFFFFFFFF8105443A, perf_tid = 20714, perf_pid = 20714, perf_period =
>>>> 114 }
>>>>     ...
>>>
>>> How is babeltrace showing time-of-day for perf-based data files? Is that
>>> tod when the command is run?
>>
>> CTF needs an a base offset which we set to 0 because we don't have it.
>> It then takes the NS timestamp and computes the "time".
> 
> so the perf-clock timestamp is converted to hour-min-second-nsec. That
> is even more confusing - for me at least.

This is the default view of babeltrace. You can switch it off and see
the timestamp in (base + ns).

> By base offset you mean the conversion between perf-clock and realtime?
yes.

> What if that information is known (e.g., my tree at
> https://github.com/dsahern/linux as time-of-day support through a klm) -
> what's the ctf function to set the base offset? Can it be changed as a
> file is processed - e.g., tracepoints capturing ntp adjustments?

Not sure I can follow here. If I'm not mistaken, the perf-clock starts
at 0 at boot time and is then incremented. If you manage to compute the
offset between perf-clock "0" and current-time at-this point then you
can use this as "base". That means, perf-timestamp + base is the
time+date when this event was captured. This base offset isn't stored
in the perf stream.
If you look at ctf_writer__setup_clock() (patch #3) it sets the offset
I refer to. This offset can be set only once and can't be adjusted.

> David

Sebastian
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