Re: [PATCH] sched: Support current clocksource handling infallback sched_clock().

From: john stultz
Date: Tue May 26 2009 - 16:41:15 EST

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?


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