linux-kernel - Re: [patch 17/17] Add a sched_clock paravirt

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Date:	Mon, 02 Apr 2007 00:06:22 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	virtualization@...ts.osdl.org, lkml <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Dan Hecht <dhecht@...are.com>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [patch 17/17] Add a sched_clock paravirt_op

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
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