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

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Date:	Tue, 26 May 2009 13:40:39 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Walleij <linus.ml.walleij@...il.com>,
	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().

On Tue, 2009-05-26 at 22:30 +0200, Peter Zijlstra wrote:
> On Tue, 2009-05-26 at 13:23 -0700, john stultz wrote:
> > Overall, I'd probably suggest thinking this through a bit more. At some
> > point doing this right will cause sched_clock() to be basically the same
> > as ktime_get(). So why not just use that instead of remaking it?
> 
> simply because we don't require the strict global monotonicy for
> scheduling as we do from a regular time source (its nice to have
> though).
> 
> That means that on x86 we can always use TSC for sched_clock(), even
> when its quite unsuitable for ktime.

Right, but I guess what I'm asking is can this be a bit better defined? 

If we are going to use clocksources (or cyclecounters - an area I need
to clean up soon), it would be good to get an idea of what is expected
of the sched_clock() interface.

So TSC good, HPET bad. Why? Is latency all we care about? How bad would
the TSC have to be before we wouldn't want to use it?

thanks
-john


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