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Subject: Re: [PATCH 2/6] Add IRQ_TIME_ACCOUNTING, finer accounting of CPU
 irq time
From: Peter Zijlstra <peterz@infradead.org>
To: Venkatesh Pallipadi <venki@google.com>
Cc: Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-kernel@vger.kernel.org, Paul Turner <pjt@google.com>
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Date: Sun, 19 Sep 2010 13:42:40 +0200
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On Sun, 2010-09-19 at 13:21 +0200, Peter Zijlstra wrote:
> On Thu, 2010-09-16 at 18:56 -0700, Venkatesh Pallipadi wrote:
> > +void account_system_vtime(struct task_struct *tsk)
> > +{
> > +       unsigned long flags;
> > +       int cpu;
> > +       u64 now, delta;
> > +
> > +       if (!sched_clock_irqtime)
> > +               return;
> > +
> > +       local_irq_save(flags);
> > +
> > +       cpu = task_cpu(tsk);
> > +       now = sched_clock();
> > +       delta = now - per_cpu(irq_start_time, cpu);
> > +       per_cpu(irq_start_time, cpu) = now;
> > +       if (hardirq_count())
> > +               per_cpu(cpu_hardirq_time, cpu) += delta;
> > +       else if (softirq_count())
> > +               per_cpu(cpu_softirq_time, cpu) += delta;
> > +
> > +       local_irq_restore(flags);
> > +} 
> 
> This seems to suggest you count time double if a hardirq hits while
> we're doing softirqs, but being as this is an incomplete api its very
> hard to tell indeed.

Argh, so this function is called on both _enter and _exit, egads.

Imagine the following in task context:

  local_bh_disable();

  <IRQ>

  local_bh_enable();

That seems to incorrectly inflate softirq_time.