linux-kernel - Re: [PATCH 2/6] Add IRQ_TIME_ACCOUNTING, finer accounting of CPU irq time

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Message-ID: <1284895291.2275.617.camel@laptop>
Date:	Sun, 19 Sep 2010 13:21:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 2/6] Add IRQ_TIME_ACCOUNTING, finer accounting of CPU
 irq time

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.
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