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

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Date:	Wed, 27 May 2009 02:18:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Paul Mundt <lethal@...ux-sh.org>
cc:	john stultz <johnstul@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	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>
Subject: Re: [PATCH] sched: Support current clocksource handling in fallback
 sched_clock().

On Wed, 27 May 2009, Paul Mundt wrote:
> > Yea. So this is a little better. There's still a few other issues to
> > consider:
> > 
> > 1) What if a clocksource is registered that has the _SCHED_CLOCK bit
> > set, but is not selected for timekeeping due it being unstable like the
> > TSC?
> > 
> See, this is what I thought the rating information was useful for, as the
> rating is subsequently dropped if it is not usable. But perhaps it makes
> more sense to just clear the bit at the same time that the rating is
> lowered once it turns out to be unstable.

Stop worrying about TSC please. The x86 f*cked up timers need special
handling which is definitely not required for most of arch/*. x86
overrides that anyway and handles the TSC f*ckup in the
CONFIG_HAVE_UNSTABLE_SCHED_CLOCK section of sched_clock.c which is
completely irrelevant to any architecture which has a sane set of
timers.

The only extra magic which is required to avoid that (sub)arch
maintainers need to specify a sched_clock() implementation to override
the weak generic one is really the simple

    if (clock && (clock->flags & CLOCKSOURCE_USE_FOR_SCHED_CLOCK))
       return ....

We need no locking there at all.

We have a workaround in place, which overrides the weak sched_clock()
implementation, to make x86 efficient, so why do we want to impose all
that x86 crap on folks which deal with architectures which got the
timers right ?

Thanks,

	tglx
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