linux-kernel - Re: [RFC] ARM: sched_clock: update epoch

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Message-ID: <20120727224128.GA14835@n2100.arm.linux.org.uk>
Date:	Fri, 27 Jul 2012 23:41:28 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	"Bedia, Vaibhav" <vaibhav.bedia@...com>,
	Colin Cross <ccross@...roid.com>,
	Nicolas Pitre <nico@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [RFC] ARM: sched_clock: update epoch_cyc on resume

On Sat, Jul 28, 2012 at 12:23:05AM +0200, Linus Walleij wrote:
> On Tue, Jul 24, 2012 at 11:16 AM, Bedia, Vaibhav <vaibhav.bedia@...com> wrote:
> 
> >> > A connecting theme is that of being avle to flag clock sources as
> >> > sched_clock providers. If all clocksources were tagged with
> >> > rating, and only clocksources were used for sched_clock(), the
> >> > kernel could select the highest-rated clock under all circumstances.
> >> >
> >> > But that's quite intrusive, more of an idea. :-P
> >>
> >
> > Too intrusive I guess ;)
> >
> > There were some discussions on this in the context of AM335x [1] [2].
> > Right now only sched_clock can be registered and I guess this restriction
> > is not going to go away any time soon.
> 
> Why do you think that? The restriction to only assign sched_clock() at
> compile-time was recently removed so its now runtime assigned.
> 
> So yes, a clock source that can die and change frequency is no good
> as primary system time, but the abstraction could still be used for
> those that do, just add another flag NOT_CONTINUOUS or so, and
> make sure the system does not select this for primary system
> clock source.
> 
> Then modelling sched_clock() on clock sources makes all more
> sense: just select the best one. For primary system clock source,
> do not select one which is non-continous.

Except for the overhead.  sched_clock() is supposed to be as _fast_ as
it can possibly be, because it has a direct impact on the performance
of the system.
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