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

[Jeremy Fitzhardinge <jeremy@goop.org>]

Andi Kleen wrote:
> Do you also get a clock for stolen nanoseconds? 
>   

What you actually get is how many ns the CPU spent in each state. 
Stolen is runnable+offline.

> No need for cycles, you could just subtract the stolen ns if you
> can get those.

It just seems like a simpler interface to just allow overriding
sched_clock, rather than exposing sched_clock's internals and fiddling
with those.  There's really nothing in the existing sched_clock which
can be profitably reused in the Xen case.  VMI can make use of the
cycles_2_ns conversion, which is why I made it available for its use.

To be specific, this is the whole Xen sched_clock function:

/* Xen sched_clock implementation.  Returns the number of RUNNING ns */
unsigned long long xen_sched_clock(void)
{
	struct vcpu_runstate_info state;
	cycle_t now;
	unsigned long long ret;

	preempt_disable();

	now = xen_clocksource_read();
	get_runstate_snapshot(&state);
	WARN_ON(state.state != RUNSTATE_running);

	ret = state.time[RUNSTATE_running] + (now - state.state_entry_time);

	preempt_enable();

	return ret;
}


    J