Re: [PATCH] U300 sched_clock implementation

Linus Walleij
2009-06-01T07:46:19Z

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> ?

Russell wanted an indication from the scheduler people that it looked sane...

Yours,
Linus Walleij