linux-kernel - Re: [PATCH] U300 sched

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Message-Id: <1243933212.23657.5726.camel@twins>
Date:	Tue, 02 Jun 2009 11:00:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Walleij <linus.ml.walleij@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-arm-kernel@...ts.arm.linux.org.uk
Subject: Re: [PATCH] U300 sched_clock implementation

On Mon, 2009-06-01 at 09:46 +0200, Linus Walleij wrote:
> 2009/5/25 Peter Zijlstra <peterz@...radead.org>:
> > On Mon, 2009-05-25 at 14:13 +0200, Linus Walleij wrote:
> >> 2009/5/24 Peter Zijlstra <peterz@...radead.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@...radead.org> ?

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