linux-kernel - Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

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Message-Id: <1173230571.24738.534.camel@localhost.localdomain>
Date:	Wed, 07 Mar 2007 02:22:50 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dan Hecht <dhecht@...are.com>
Cc:	Zachary Amsden <zach@...are.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, ak@...e.de,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>
Subject: Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

On Tue, 2007-03-06 at 16:42 -0800, Dan Hecht wrote:
> >> accounting would be wrong.  Instead, we should allow the 
> >> tick_sched_timer in cases (c) and (d) to have runtime configurable 
> >> period, and then scale the time value accordingly before passing to 
> >> account_system_time.  This is probably something the Xen folks will want 
> >> also, since I think Xen itself only gets 100hz hard timer, and so it can 
> >> implement at best a oneshot virtual timer with 100hz resolution.  Any 
> >> objections to us doing something like this?
> > 
> > Yes. It's gross hackery. 
> > 
> > 1) We want to have a cleanup of the tick assumptions _all_ over the
> > place and this is going to be real hard work.
> > 
> > 2) As I said above. The time accounting for virtualization needs to be
> > fixed in a generic way.
> > 
> > I'm not going to accept some weird hackery for virtualization, which is
> > of exactly ZERO value for the kernel itself. Quite the contrary it will
> > make the cleanup harder and introduce another hard to remove thing,
> > which will in the worst case last for ever.
> >
> 
> Okay, to confirm I'm on the same page as you, you want to move process 
> time accounting from being periodic sampled based to being trace based? 
> i.e. at the system-call/interrupt boundaries, read clocksource and 
> compute directly the amount of system/user/process time?

At least for the paravirt guests this is the correct approach. Once the
CPU vendors come up with a sane solution for a reliable and fast clock
source we might use that on real hardware as well.

> Do you know if anyone has explored this?  I thought there was a 
> discussion about this a while back but it was rejected due to the 
> sample-based approach having much lower overheads on high system call 
> rate workloads.

Yes, with todays hardware it is simply a PITA. PowerPC has some basic
support for this though, IIRC.

	tglx


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