linux-kernel - Re: [PATCH, RFC] use hrtimer in sched

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Date:	Mon, 14 Jul 2008 11:48:47 +0200
From:	Uwe Kleine-König <Uwe.Kleine-Koenig@...i.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH, RFC] use hrtimer in sched_clock

Hello Thomas,

Thomas Gleixner wrote:
> On Fri, 11 Jul 2008, Uwe Kleine-König wrote:
> > This should make it unnecessary to overwrite sched_clock for a higher
> > precision.
> > With this patch I get sub-jiffie timing with CONFIG_PRINTK_TIME=y.
> >
> > Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@...i.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > ---
> > Hello,
> >
> > I tested the patch on arch-arm/mach-ns9xxx and it seem seams to work.
> > But I admit I didn't test it deeply and I didn't measure if there is any
> > overhead.
> 
> There is lots of overhead and your approach is simply wrong.
I expected to have some overhead, but can you please elaborate on
"simply wrong"?
I will try to measure the actual overhead but cannot promise to find
time for that in the near future.
 
> >  unsigned long long __attribute__((weak)) sched_clock(void)
> 
> The correct way to solve it is to override sched_clock() with a high
> resolution implementation for your hardware platform. sched_clock is a
> weak function to provide a default implementation based on jiffies.
> 
> If you do a grep -r sched_clock arch/arm you'll find a couple of
> examples how to override sched_clock().
I new that and my purpose was to make this unneeded.  Would be nice,
wouldn't it?

Thanks and best regards,
Uwe

-- 
Uwe Kleine-König, Software Engineer
Digi International GmbH Branch Breisach, Küferstrasse 8, 79206 Breisach, Germany
Tax: 315/5781/0242 / VAT: DE153662976 / Reg. Amtsgericht Dortmund HRB 13962
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