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Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().
From: Peter Zijlstra <peterz@infradead.org>
To: john stultz <johnstul@us.ibm.com>
Cc: Linus Walleij <linus.ml.walleij@gmail.com>,
       Paul Mundt <lethal@linux-sh.org>, Ingo Molnar <mingo@elte.hu>,
       Andrew Victor <linux@maxim.org.za>,
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       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
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       Thomas Gleixner <tglx@linutronix.de>
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Date: Wed, 27 May 2009 08:58:46 +0200
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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.