linux-kernel - Re: [PATCH v5] perf: Use monotonic clock as a source for timestamps

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Message-ID: <20150212102814.GK2896@worktop.programming.kicks-ass.net>
Date:	Thu, 12 Feb 2015 11:28:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Pawel Moll <pawel.moll@....com>, ajh mls <ajhmls@...il.com>,
	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>,
	John Stultz <john.stultz@...aro.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 v5] perf: Use monotonic clock as a source for timestamps

On Thu, Feb 12, 2015 at 12:04:54PM +0200, Adrian Hunter wrote:
> On 11/02/15 18:12, Peter Zijlstra wrote:
> > 
> > How about something like the below? I _think_ it should mostly work for
> > x86, where the tsc is a 64bit wide cycle counter.
> 
> It would have to be based on CLOCK_MONOTONIC_RAW not CLOCK_MONOTONIC 

Why?

> and you would have to check the clocksource is TSC.

It implicitly does that; it has that sched_clock_stable() thing, but
yeah I suppose someone could change the clocksource even though the tsc
is stable.

Not using TSC when its available is quite crazy though.. but sure.

> Why is CLOCK_MONOTONIC preferred anyway - I would have thought any
> adjustment would skew performance timings?

Because you can do inter-machine stuff with MONOTONIC and that's
entirely impossible with MONO_RAW.
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