linux-kernel - Re: [RFC] sched_clock: Track monotonic raw clock

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Date:	Fri, 18 Jul 2014 21:13:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pawel Moll <pawel.moll@....com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	John Stultz <john.stultz@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Baruch Siach <baruch@...s.co.il>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched_clock: Track monotonic raw clock

On Fri, Jul 18, 2014 at 06:43:39PM +0100, Pawel Moll wrote:
> This change is trying to make the sched clock "similar" to the
> monotonic raw one.
> 
> The main goal is to provide some kind of unification between time
> flow in kernel and in user space, mainly to achieve correlation
> between perf timestamps and clock_gettime(CLOCK_MONOTONIC_RAW).
> This has been suggested by Ingo and John during the latest
> discussion (of many, we tried custom ioctl, custom clock etc.)
> about this:
> 
> http://thread.gmane.org/gmane.linux.kernel/1611683/focus=1612554
> 
> For now I focused on the generic sched clock implementation,
> but similar approach can be applied elsewhere.
> 
> Initially I just wanted to copy epoch from monotonic to sched
> clock at update_clock(), but this can cause the sched clock
> going backwards in certain corner cases, eg. when the sched
> clock "increases faster" than the monotonic one. I believe
> it's a killer issue, but feel free to ridicule me if I worry
> too much :-)

But on hardware using generic sched_clock we use the exact same hardware
as the regular timekeeping, right?

So we could start off with the same offset/mult/shift and never deviate,
or is that a silly question?, I've never really looked at the generic
sched_clock stuff too closely.
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