linux-kernel - Re: [PATCH] Allow preemption during lazy mmu updates

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Date:	Sun, 19 Apr 2009 12:47:25 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Avi Kivity <avi@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Allow preemption during lazy mmu updates

On Sun, 2009-04-19 at 13:15 +0300, Avi Kivity wrote:
> Peter Zijlstra wrote:
> > On Wed, 2009-04-08 at 16:54 +0200, Ingo Molnar wrote:
> >   
> >>>  kernel/sched.c                     |    2 -
> >>>       
> >> Needs the ack of ... oh, never mind - this one is fine i guess ;-)
> >>     
> >
> > Ah, about that. This new preemption hook has slightly different
> > requirements than the current preempt-notifiers have (hence the new
> > hook), I was wondering if KVM (afaik currently the only preempt-notifier
> > consumer) could live with these requirements.
> >
> > That is, could these be merged?
> >   
> 
> What are the slight differences in requirements?
> 
> KVM wants to run in non-preemptible, interrupts-enabled context.

The fire_sched_out bit is a little earlier, but I don't think that is a
particularly worrysome, but the most important difference was that
fire_sched_in in far too late. arch_end_context_switch() is done right
in the middle of switch_to() because it needs the TS bit or somesuch.

I'll let Jeremy explain details, as I've long since forgotten them :-)

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