Re: [PATCH] U300 sched_clock implementation

Peter Zijlstra
2009-06-02T09:00:24Z

On Mon, 2009-06-01 at 09:46 +0200, Linus Walleij wrote:
> 2009/5/25 Peter Zijlstra <peterz@infradead.org>:
> > On Mon, 2009-05-25 at 14:13 +0200, Linus Walleij wrote:
> >> 2009/5/24 Peter Zijlstra <peterz@infradead.org>:
> >>
> >> > On Sat, 2009-05-23 at 23:46 +0200, Linus Walleij wrote:
> >> >
> >> >> This overrides the global sched_clock() symbol in the Linux
> >> >> scheduler with a local implementation which takes advantage of
> >> >> the timesource in U300 giving a scheduling resolution of 1us. The
> >> >> solution is the same as found in the OMAP2 core code.
> >> >
> >> > We assume sched_clock() to return time in ns (e-9) resolution.
> >>
> >> Yep okay and in this case:
> >>
> >> >> + ret = (unsigned long long) u300_get_cycles();
> >> >> + ret = (ret * clocksource_u300_1mhz.mult_orig) >>
> >> >> + clocksource_u300_1mhz.shift;
> >> >> + return ret;
> >>
> >> (mult_orig >> shift) == 1000
> >
> > Ah, ok -- missed that little detail ;-)
> >
> >> So for each cycle in cyclecount register we return 1000 * cycles
> >> i.e 1000ns.
> >>
> >> If it looks nicer we can of course simply:
> >> return (unsigned long long) u300_get_cycles * 1000;
> >>
> >> But the question here is whether this resolution is enough for
> >> sched_clock() or if it is irrelevant to override sched_clock()
> >> if it cannot schedule with better precision than 1000 ns.
> >
> > No anything better than jiffies is good, 1us certainly is worth the
> > trouble.
>
> Can I interpret this as Acked-by: Peter Zijlstra <peterz@infradead.org> ?

I think its best if we continue with the patch Paul Mundt has been
proposing.