linux-kernel - Re: Stolen and degraded time and schedulers

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Date:	Tue, 13 Mar 2007 14:27:04 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	john stultz <johnstul@...ibm.com>, Andi Kleen <ak@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Con Kolivas <kernel@...ivas.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zachary Amsden <zach@...are.com>,
	James Morris <jmorris@...ei.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	cpufreq@...ts.linux.org.uk,
	Virtualization Mailing List <virtualization@...ts.osdl.org>
Subject: Re: Stolen and degraded time and schedulers

On Tue, 2007-03-13 at 13:32 -0700, Jeremy Fitzhardinge wrote:

> Most of the existing clocksource infrastructure would only operate on
> CLOCK_TIMEBASE_REALTIME clocksources, so I'm not sure how much overlap
> there would be here.  In the case of dealing with cpufreq, there's a
> certain appeal to manipulating the shift/mult parameters to reflect the
> fractional speed of a cpu as it changes.

The frequency tracking you mention is done to some extent inside the
timekeeping adjustment functions, but I'm not sure it's totally accurate
for non-timekeeping, and it also tracks things like interrupt latency.
Tracking frequency changes where it's important to get it right
shouldn't be done I think ..

If you want accurate time accounting, don't use the TSC .

> sched_clock would definitely be the interface which exposes all this
> stuff to the rest of the kernel.  After all, its basically a very simple
> interface, though the backend implementation details may not be.

The sched_clock interface is basically a stripped down clocksource..
I've implemented sched_clock as a clocksource in the past ..

> We currently have a sched_clock interface in paravirt_ops to deal with
> the hypervisor aspect.  It only occurred to me this morning that cpufreq
> presents exactly the same problem to the rest of the kernel, and so
> there's room for a more general solution.

Are there other architecture which have this per-cpu clock frequency
changing issue? I worked with several other architectures beyond just
x86 and haven't seen this issue ..

Daniel

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