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

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Date:	Mon, 2 Apr 2007 08:09:42 +0200
From:	Andi Kleen <ak@...e.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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

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
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