linux-kernel - Re: [PATCH v2 5/6] KVM-GST: adjust scheduler cpu power

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Date:	Tue, 01 Feb 2011 17:55:29 -0200
From:	Glauber Costa <glommer@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH v2 5/6] KVM-GST: adjust scheduler cpu power

On Tue, 2011-02-01 at 19:59 +0100, Peter Zijlstra wrote:
> On Tue, 2011-02-01 at 14:22 -0200, Glauber Costa wrote:
> > 
> > 
> > Which tick accounting? In your other e-mail , you pointed that this only
> > runs in touch_steal_time, which is fine, will change.
> 
> That tick ;-), all the account_foo muck is per tick.
> 
> >  But all the rest
> > here, that is behind the hypervisor specific vs generic code has nothing
> > to do with ticks at all.
> 
> But I don't get it, there is no generic code needed, all that's needed
> is u64 steal_time_clock(int cpu), and the first part of your
> kvm_account_steal_time() function is exactly that if you add the cpu
> argument.
> 
> +static u64 steal_time_clock(int cpu)
> +{
> +       u64 steal_time;
> +       struct kvm_steal_time *src;
> +       int version;
> +
> +	preempt_disable();
> +       src = &per_cpu_ptr(steal_time, cpu);
> +       do {
> +               version = src->version;
> +               rmb();
> +               steal_time = src->steal;
> +               rmb();
> +       } while ((src->version & 1) || (version != src->version));
> +       preempt_enable();
> +
> +       return steal_time
> +}
> 
> And you're done.. no need to for any of that steal_time_{read,write} business.

update_rq_clock_task still have to keep track of what was the last steal
time value we saw, in the same way it does for irq. One option is to
call update_rq_clock_task from inside kvm-code, but I don't really like
it very much.

But okay, there are many ways to work around it, I'll cook something.

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