Re: Stolen and degraded time and schedulers

On Tue, 2007-03-13 at 13:32 -0700, Jeremy Fitzhardinge wrote:

Most of the existing clocksource infrastructure would only operate on
CLOCK_TIMEBASE_REALTIME clocksources, so I'm not sure how much overlap
there would be here. In the case of dealing with cpufreq, there's a
certain appeal to manipulating the shift/mult parameters to reflect the
fractional speed of a cpu as it changes.

The frequency tracking you mention is done to some extent inside the
timekeeping adjustment functions, but I'm not sure it's totally accurate
for non-timekeeping, and it also tracks things like interrupt latency.
Tracking frequency changes where it's important to get it right
shouldn't be done I think ..

If you want accurate time accounting, don't use the TSC .

sched_clock would definitely be the interface which exposes all this
stuff to the rest of the kernel. After all, its basically a very simple
interface, though the backend implementation details may not be.

The sched_clock interface is basically a stripped down clocksource..
I've implemented sched_clock as a clocksource in the past ..

We currently have a sched_clock interface in paravirt_ops to deal with
the hypervisor aspect. It only occurred to me this morning that cpufreq
presents exactly the same problem to the rest of the kernel, and so
there's room for a more general solution.

Are there other architecture which have this per-cpu clock frequency
changing issue? I worked with several other architectures beyond just
x86 and haven't seen this issue ..

Daniel

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