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

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Message-Id: <1243348681.23657.14.camel@twins>
Date:	Tue, 26 May 2009 16:38:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Walleij <linus.ml.walleij@...il.com>
Cc:	Paul Mundt <lethal@...ux-sh.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	John Stultz <johnstul@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().

Added the generic clock and timer folks to CC.

On Tue, 2009-05-26 at 16:31 +0200, Linus Walleij wrote:
> 2009/5/26 Paul Mundt <lethal@...ux-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?




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