linux-kernel - Re: [PATCH v3 8/9] KVM-GST: adjust scheduler cpu power

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Date:	Thu, 30 Jun 2011 23:54:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH v3 8/9] KVM-GST: adjust scheduler cpu power

On Wed, 2011-06-29 at 11:29 -0400, Glauber Costa wrote:

> @@ -2063,12 +2092,7 @@ static int irqtime_account_si_update(void)
>  
>  #define sched_clock_irqtime    (0)
>  
> -static void update_rq_clock_task(struct rq *rq, s64 delta)
> -{
> -       rq->clock_task += delta;
> -}
> -
> -#endif /* CONFIG_IRQ_TIME_ACCOUNTING */ 

That relies on the compiler realizing that irq_delta and steal will
never be used in the below function:

> static void update_rq_clock_task(struct rq *rq, s64 delta)
>  {
> +       s64 irq_delta = 0, steal = 0;
>  
> +#ifdef CONFIG_IRQ_TIME_ACCOUNTING
>         irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
>  
>         /*
> @@ -2023,12 +2034,30 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>  
>         rq->prev_irq_time += irq_delta;
>         delta -= irq_delta;
> +#endif
> +#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> +       if (static_branch((&paravirt_steal_rq_enabled))) {
> +               int is_idle;
> +               u64 st;
> +
> +               is_idle = ((rq->curr != rq->idle) ||
> +                               irq_count() != HARDIRQ_OFFSET);
> +
> +               __touch_steal_time(is_idle, delta, &st);
> +
> +               steal = st * TICK_NSEC;
> +
> +               delta -= steal;
> +       }
> +#endif
> +
>         rq->clock_task += delta;
>  
> +       if ((irq_delta + steal) && sched_feat(NONTASK_POWER))
> +               sched_rt_avg_update(rq, irq_delta + steal);
>  }

And thus trim the last bit as if(0) and DCE. Did you verify that the
compiler does indeed do that for !PARAVIRT !IRQ_TIME_ACCOUNTING ?
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