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

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Message-ID: <20140719050233.GA4408@netboy>
Date:	Sat, 19 Jul 2014 07:02:33 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Pawel Moll <pawel.moll@....com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 code definitely needs more work and testing (I'm not 100%
> sure if the Kp and Ki I've picked for the proportional and
> integral terms are universal),

I wouldn't bet on it.

> but for now wanted to see
> if this approach makes any sense whatsoever.

You are reading sched_clock and mono-raw together every so
often. Really stupid question: Why not just place that information
into the trace buffer and let user space do the clock correction?

...

> +		/* Tune the cyc_to_ns formula */
> +		mult_adj = sign * (error >> 2) + (cd.error_int >> 2);

So Kp = Ki = 0.25? And did you say that the sample rate is 10/second?

I guess that, while this works well on your machine, it might not
always do so, depending on the mono-raw clock. Probably Kp/i need to
be tunable to a particular system. Even better would be to leave this
out of the kernel altogether.

Thanks,
Richard
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