linux-kernel - Re: [PATCH] ftrace: Provide trace clock monotonic raw

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Message-ID: <20150504205748.GB21418@twins.programming.kicks-ass.net>
Date:	Mon, 4 May 2015 22:57:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Drew Richardson <drew.richardson@....com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Wade Cherry <Wade.Cherry@....com>,
	Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH] ftrace: Provide trace clock monotonic raw

On Mon, May 04, 2015 at 01:05:19PM -0700, Drew Richardson wrote:
> I'm collecting and merging data from perf, with Android Atrace data
> (writes to /sys/kernel/debug/tracing/trace_marker) which ends up in
> the ftrace stream and other measurements collected from
> userspace. Currently the only clock readable from userspace, supported
> by perf and by ftrace is CLOCK_MONOTONIC. However this clock is
> affected by the incremental adjustments performed by adjtime(3) and
> NTP. 

Which should not matter at all, right? If both sources are using the
same clock (they are) then its trivial to merge them and everything
works as expected.

> But I'd prefer to use a clock that is advancing at a consistent
> rate, hence CLOCK_MONOTONIC_RAW.

Right, Mathieu is asking _why_ you prefer that?
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