Re: [patch 17/17] Add a sched_clock paravirt_op

Andi Kleen
2007-04-02T06:09:57Z

On Monday 02 April 2007 07:57, Jeremy Fitzhardinge wrote:
> The tsc-based get_scheduled_cycles interface is not a good match for
> Xen's runstate accounting, which reports everything in nanoseconds.
>
> This patch replaces this interface with a sched_clock interface, which
> matches both Xen and VMI's requirements.
>
> In order to do this, we:
> 1. replace get_scheduled_cycles with sched_clock
> 2. hoist cycles_2_ns into a common header
> 3. update vmi accordingly
>
> One thing to note: because sched_clock is implemented as a weak
> function in kernel/sched.c, we must define a real function in order to
> override this weak binding. This means the usual paravirt_ops
> technique of using an inline function won't work in this case.

I think it would be much cleaner if you didn't implement your own sched_clock,
but you adjust ns_base/last_tsc to account for your lost cycles.
This could be done cleanly by adding a new function to sched-clock.c
Possibly such a function could be used by other parts of the kernel
in the future too.

-Andi