On Fri, 14 Jun 2013, Ming Lei wrote:

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With the two trace points of irq_handler_entry and irq_handler_exit, the
interrupt latency(or the time taken by hard irq handler) isn't hard to measure.
One simple script can figure out the average/maximum latency for one irq
handler, like I did in 4/4.

But that doesn't measure the time between when the IRQ request is
issued and when irq_handler_entry runs.

That might be hard to measure, also I am wondering if the time can be
measured only by software, but these patches only focus on the time
between HCD irq entry and irq exit.

Not entirely.  On a UP system, leaving interrupts disabled for a long
time (which is what we do now) increases the delay between when the IRQ
is raised and when it is serviced.  On an SMP system, a long-running 
interrupt handler will delay servicing a different device that shares 
the same IRQ line.

Alan Stern