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Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().
From: Peter Zijlstra <peterz@infradead.org>
To: Paul Mundt <lethal@linux-sh.org>
Cc: Linus Walleij <linus.ml.walleij@gmail.com>, Ingo Molnar <mingo@elte.hu>,
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Date: Tue, 26 May 2009 16:50:09 +0200
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On Tue, 2009-05-26 at 23:43 +0900, Paul Mundt wrote:
> > 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?
> > 
> This rather defeats the purpose of sched_clock() being fast. If we want
> to add a flag that means this in to the clocksource instead of consulting
> the rating, that is fine with me too. I know which clocksources I prefer
> to use for a sched_clock() and they are all better than jiffies. The
> semantics of how we tell sched_clock() that are not so important. Rating
> seemed like a good choice from the documentation in struct clocksource at
> least.

Am I confused or are we talking about fast HZ vs fast cycles?

sched_clock() should be fast cycles, that is, we don't want to read a
clock that takes about 1000 cycles.

sched_clock() is about providing a high resolution clock that is fast
(low cycle count) to acquire, and need not be strictly monotonic on smp.