linux-kernel - Re: [PATCH v2 1/2] perf: Add sampling of the raw monotonic clock

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Message-ID: <1411742306.1669.6.camel@leonhard>
Date:	Fri, 26 Sep 2014 23:38:26 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Pawel Moll <pawel.moll@....com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] perf: Add sampling of the raw monotonic clock

2014-09-26 (금), 11:58 +0100, Pawel Moll:
> On Fri, 2014-09-26 at 07:16 +0100, Namhyung Kim wrote:
> > > It would be doable, I guess, but what
> > > if someone *wants* to have sched clock as the timestamps source (because
> > > it's cheap) but still be able to correlate them with userspace? In this
> > > case two separate timestamps are required to do the approximation.
> > 
> > But by collecting two timestamps, you'll loose the win of the first
> > timestamp, no?
> 
> But I can ask for both timestamps only being collected on "low
> bandwidth" events, in particular context switches and/or periodic (eg.
> 10ms hrtimer) software events.
> 
> Then I have loads of normal normal samples, timestamped with sched clock
> only, and every now and then one with both timestamps which then I can
> use for time correlation. The whole point is that the frequency of such
> "synchronisation" event can be much (much!) lower than of the normal
> samples, but it still allows pretty good approximation (I was getting
> accuracy of ~1 microsecond and better with sched_switch trace event
> marked with additional raw monotonic timestamp).

Okay.  But in that case wouldn't it be enough to use just a single
timestamp for each event - sched_clock for cpu-cycles and monotonic raw
for sched_switch?


> 
> Pawel
> 
> PS. Have you sent a couple of the messages via some kind of gmane's
> proxy? All the mail addresses got rather messed up...
> 

Yes, I use gnus to communicate with lkml, but it seems it messed up mail
addresses if it contains some list (like linux-api ?).  Sorry about
that.

Thanks,
Namhyung


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