Messages in this thread |  | | | Date | Tue, 26 May 2009 23:53:59 +0900 | | From | Paul Mundt <> | | Subject | Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock(). |
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On Tue, May 26, 2009 at 04:50:09PM +0200, Peter Zijlstra wrote: > 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.
I don't think there's any confusion here. My point is that I didn't want to add too much logic in to sched_clock() given that it is supposed to be fast. So if the rating test by itself is not sufficient, then we need another way to flag a clocksource as being usable for sched_clock().
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