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

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Date:	Tue, 22 Jul 2014 17:48:01 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Richard Cochran <richardcochran@...il.com>,
	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" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] sched_clock: Track monotonic raw clock

On Tue, 2014-07-22 at 17:34 +0100, Peter Zijlstra wrote:
> On Tue, Jul 22, 2014 at 05:17:29PM +0100, Pawel Moll wrote:
> > With both Ingo and John showing preference towards the clock alignment,
> > so that's where I looked this time (I've already done custom perf
> > ioctls, posix clocks... don't really know how many different ways I've
> > tried).
> 
> So we should probably also talk about which clock to track, MONO has the
> advantage of making it far easier to trace clusters but has the
> disadvantage of stacked control loops with NTP adjusting MONO and us
> adjusting sched_clock.

John suggested (and I fully agree with him) MONO_RAW, as it is not
getting NTP-d. 

> And I would really prefer to pick 1 and not make it configurable.

Same here. One thing I keep in mind is the fact that userspace must be
able to say whether it can expect the correlation or not. "Not" being
either an architecture which sched_clock is not using the generic
solution (I'm sure there will be some) or "not" because of the
synchronisation failure. My idea so far was a debugfs file saying this
(or missing, which is a message on its own).

Pawel


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