Re: [PATCH] sched: Support current clocksource handling in fallback

[Thomas Gleixner <tglx@linutronix.de>]

John,

On Tue, 26 May 2009, john stultz wrote:
> > See, this is what I thought the rating information was useful for, as the
> > rating is subsequently dropped if it is not usable. But perhaps it makes
> > more sense to just clear the bit at the same time that the rating is
> > lowered once it turns out to be unstable.
> 
> Yes, if we're dropping a clocksource we should also drop the bit. That
> shouldn't be a problem.
> 
> The point I was making, is that multiple clocksources may be registered
> at one time (TSC, ACPI_PM, etc). But only one is being managed by the
> timekeeping code (clock). So there may be the case where the
> sched_clock() is different then the timekeeping clock (which is common
> on x86). 
> 
> So I suspect we need a special hook that grabs the best _SCHED_CLOCK
> clocksource (as computed at clocksource registration time) and provides
> it to the generic sched_clock() interface.

this is not about x86 and its inferiour timer hardware
implementation. We talk about sane architectures which do not have
that problems at all. x86 takes a different code path and overrides
the generic weak sched_clock implememtation anyway. So what ?

Thanks,

	tglx