Date: Thu, 12 Feb 2015 11:28:14 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Adrian Hunter <adrian.hunter@intel.com>
Cc: Pawel Moll <pawel.moll@arm.com>, ajh mls <ajhmls@gmail.com>,
        Richard Cochran <richardcochran@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Paul Mackerras <paulus@samba.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Christopher Covington <cov@codeaurora.org>,
        Namhyung Kim <namhyung@kernel.org>, David Ahern <dsahern@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tomeu Vizoso <tomeu@tomeuvizoso.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>
Subject: Re: [PATCH v5] perf: Use monotonic clock as a source for timestamps
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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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