What is the difference between /proc/interrupts and /proc/softirq in Linux?

Question

/proc/softirq is softirq stats. Is /proc/interrupt both hard and soft interrupts or hard only?

I want to measure the rate of hard and soft irq's per second roughly using watch -n 1 grep 'foo' /proc/softirq and watch -n 1 grep 'bar' /proc/interrupt so I can compare the rate of hardware interrupt increase to software interrupt.

I'm wondering if I need to subtract /proc/softirq counts from /proc/interrupt to get the count of hardware IRQs because it counts both kinds or if /proc/interrupt is hardware only?

Answer 1

softirqs aren't directly related to hardware interrupts, they're the successor to "bottom halves" and the predecessor of tasklets. The (old) Unreliable Guide to Hacking the Linux Kernel has a brief section on the topic; I dare say there are better resources elsewhere. The list of softirqs is defined in include/linux/interrupt.h; you'll see they don't correspond to single hardware interrupts.

Thus you shouldn't subtract /proc/softirq counts from /proc/interrupts. The latter only counts hardware interrupts; these of course may result in softirqs being used too, but there's no easy way of determining the correlation (e.g. between hardware interrupts on your network adapter and NET_RX or NET_TX softirqs).