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

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Date:	Wed, 27 May 2009 08:58:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	john stultz <johnstul@...ibm.com>
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 16:00 -0700, john stultz wrote:

> Sure, I guess what I'm trying to pull out here is that should we try to
> create some OK_FOR_SCHED_CLOCK flag for clocksources, and then we try to
> make this generic so other arches can add that flag and be done, what is
> the guidance we want to give to arch maintainers for setting that flag?
> 
> 1) Has to be very very fast. Can we put a number on this? 50ns to read?

I'd express it in cpu cycles, but not sure, the very fastest the arch
has :-) Just tell people this is in their scheduling hot-path and they
might understand.

> 2) How long does it have to be monotonic for? 

Forever? (per cpu)

> Is it ok if it wraps every few seconds?

No, if it wraps it needs to wrap on u64.

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