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

[Linus Walleij <linus.ml.walleij@gmail.com>]

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?

Yours,
Linus Walleij