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

From: Peter Zijlstra
Date: Tue May 26 2009 - 16:56:20 EST

On Tue, 2009-05-26 at 13:40 -0700, john stultz wrote:
> 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?

Because TSC is a few cycles to read, and you can factorize a largish
prime while doing an HPET read :-)

> Is latency all we care about? How bad would
> the TSC have to be before we wouldn't want to use it?

Anything better than jiffies ;-)

For sched_clock() we want something high-res that is monotonic per cpu
and has a bounded drift between cpus in the order of jiffies.

Look at kernel/sched_clock.c for what we do to make really shitty TSC
conform to the above requirements.

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