[patch 00/16] arm: Replace arm sched_clock by clocksource based sched_clock

[tglx@linutronix.de (Thomas Gleixner)]

On Fri, 29 Apr 2011, Russell King - ARM Linux wrote:

> On Sat, Apr 23, 2011 at 08:54:39PM -0000, Thomas Gleixner wrote:
> > Most of ARM sched_clocks is using the timekeeping clocksource as the
> > sched_clock. Only versatile uses a different clock for it, but there
> > is no real reason why it can't use the clocksource as well.
> 
> sched_clock really needs to be setup earlier than clocksources are setup,
> before sched_init() is called in init/main.c.  This is where the scheduler
> reads the initial value of sched_clock() to start its time measurements.
> This is well before clocksources are initialized, or even the irq subsystem.
> 
> So I don't believe bolting it directly into the clocksource code in this
> way is correct.  As things are currently setup with my sched_clock()
> implementation, we can (and in any cases do) get the sched_clock() up
> and running really early which is what's required.

Not really. There is no requirement to have a high resolution sched
clock in the early boot process. The only requirement is that
sched_clock is monotonic increasing nsecs. So the code utilizes
jiffies up to the point where the clocksource is installed and then
switches over keeping the clock monotonic. The scheduler does not care
whether it gets coarse grained time information in the early boot
stage and then fine grained information at some point later.

The only drawback is that you don't get precise timestamps for tracing
up to the point where the clock source is installed. But that's really
not a big deal to worry about.

Thanks,

	tglx