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

Peter Zijlstra
2009-05-26T14:38:01Z

Added the generic clock and timer folks to CC.

On Tue, 2009-05-26 at 16:31 +0200, Linus Walleij wrote:
> 2009/5/26 Paul Mundt <lethal@linux-sh.org>:
>
> > */
> > unsigned long long __attribute__((weak)) sched_clock(void)
> > {
> > + /*
> > + * Use the current clocksource when it becomes available later in
> > + * the boot process, and ensure that it has a high enough rating
> > + * to make it suitable for general use.
> > + */
> > + if (clock && clock->rating >= 100)
> > + return cyc2ns(clock, clocksource_read(clock));
> > +
> > + /* Otherwise just fall back on jiffies */
> > return (unsigned long long)(jiffies - INITIAL_JIFFIES)
> > * (NSEC_PER_SEC / HZ);
> > }
>
> This seems like it would make the patch I sent the other day
> unnecessary (subject u300 sched_clock() implementation).
>
> It would also trim off this solution found in all OMAP platforms in
> arch/arm/plat-omap/common.c
>
> BUT Peter Zijlstra replied to my question about why this wasn't
> generic with:
>
> [peterz]:
> > But that is the reason this isn't generic, non of the 'stable'
> > clocksources on x86 are fast enough to use as sched_clock.
>
> Does that mean clock->rating for these clocksources is
> for certain < 100?
>
> The definition of "rating" from the kerneldoc does not
> seem to imply that, it's a subjective measure AFAICT.
>
> Else you might want an additional criteria, like
> cyc2ns(1) (much less than) jiffies_to_usecs(1)*1000
> (however you do that the best way)
> so you don't pick something
> that isn't substantially faster than the jiffy counter atleast?