linux-kernel - Re: Make kstat_incr_irqs_this_cpu() called when an irq is actually being handled

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Date:	Thu, 31 May 2012 18:41:54 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ning Jiang <ning.n.jiang@...il.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: Make kstat_incr_irqs_this_cpu() called when an irq is actually
 being handled

On Thu, 31 May 2012, Ning Jiang wrote:
> kstat_incr_irqs_this_cpu() is supposed to record the irq number
> that is actually taken by a certain cpu. But it failed to do so
> in its current form.

It exactly counts the number of interrupts which are delivered to and
handled by a CPU. That includes interrupts which are delivered in a
state where the device handler cannot be called. And this is on
purpose. We want to see the real number of interruptions not the
number of calls to a device handler.

> For level irq, if its disabled or no action available, we'll skip
> irq handling. When it is enabled later, the irq will be retriggered
> and kstat_incr_irqs_this_cpu() will be counted twice for this
> single irq.

This counts the number of interrupts arrived at the CPU and therefor
it counts TWO in this situation. The CPU is interrupted twice, not
once.

Thanks,

	tglx

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