linux-kernel - Re: [PATCH v3 7/9] KVM-GST: KVM Steal time accounting

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Message-ID: <1309470872.12449.609.camel@twins>
Date:	Thu, 30 Jun 2011 23:54:32 +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 7/9] KVM-GST: KVM Steal time accounting

On Wed, 2011-06-29 at 11:29 -0400, Glauber Costa wrote:
> +static noinline bool touch_steal_time(int is_idle)

That noinline is very unlucky there,

> +{
> +       u64 steal, st = 0;
> +
> +       if (static_branch(&paravirt_steal_enabled)) {
> +
> +               steal = paravirt_steal_clock(smp_processor_id());
> +
> +               steal -= this_rq()->prev_steal_time;
> +
> +               st = steal_ticks(steal);
> +               this_rq()->prev_steal_time += st * TICK_NSEC;
> +
> +               if (is_idle || st == 0)
> +                       return false;
> +
> +               account_steal_time(st);
> +               return true;
> +       }
> +       return false;
> +}
> +
>  static void update_rq_clock_task(struct rq *rq, s64 delta)
>  {
>         s64 irq_delta;
> @@ -3716,6 +3760,9 @@ void account_user_time(struct task_struct *p,
> cputime_t cputime,
>         struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
>         cputime64_t tmp;
>  
> +       if (touch_steal_time(0))
> +               return; 

Means we have an unconditional call here, even if the static_branch() is
patched out.
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