linux-kernel - Re: [PATCH 16/16] KVM-GST: adjust scheduler cpu power

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Date:	Wed, 26 Jan 2011 10:57:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
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 16/16] KVM-GST: adjust scheduler cpu power

On Tue, 2011-01-25 at 19:27 -0200, Glauber Costa wrote:
> On Tue, 2011-01-25 at 22:07 +0100, Peter Zijlstra wrote:
> > On Tue, 2011-01-25 at 18:47 -0200, Glauber Costa wrote:
> > > On Tue, 2011-01-25 at 21:13 +0100, Peter Zijlstra wrote:
> > > > On Tue, 2011-01-25 at 18:02 -0200, Glauber Costa wrote:
> > > > 
> > > > > I fail to see how does clock_task influence cpu power.
> > > > > If we also have to touch clock_task for better accounting of other
> > > > > stuff, it is a separate story.
> > > > > But for cpu_power, I really fail. Please enlighten me.
> > > > 
> > > > static void update_rq_clock_task(struct rq *rq, s64 delta)
> > > > {
> > > >         s64 irq_delta;
> > > > 
> > > >         irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
> > > > 
> > > >         if (irq_delta > delta)
> > > >                 irq_delta = delta;
> > > > 
> > > >         rq->prev_irq_time += irq_delta;
> > > >         delta -= irq_delta;
> > > >         rq->clock_task += delta;
> > > > 
> > > >         if (irq_delta && sched_feat(NONIRQ_POWER))
> > > >                 sched_rt_avg_update(rq, irq_delta);
> > > > }
> > > > 
> > > > its done through that sched_rt_avg_update() (should probably rename
> > > > that), it computes a floating average of time not spend on fair tasks.
> > > > 
> > > It creates a dependency on CONFIG_IRQ_TIME_ACCOUNTING, though.
> > > This piece of code is simply compiled out if this option is disabled.
> > 
> > We can pull this bit out and make the common bit also available for
> > paravirt.
> 
> scale_rt_power() seems to do the right thing, but all the path leading
> to it seem to work on rq->clock, rather than rq->clock_task.

Not quite, see how rq->clock_task is irq_delta less than the increment
to rq->clock? You want it to be your steal-time delta less too.

> Although I do can experiment with that as well, could you please
> elaborate on what are your reasons to prefer this over than variations
> of the method I proposed?

Because I want rq->clock_task to not include steal-time.
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