printk inside an interrupt handler , is it really that bad?

Question

everybody knows that interrupt handler should be short as possible. and adding functions like printk for debugging inside an interrupt handler is something that shouldn't be done. Actually, I tried it before when I was debugging the linux kernel for an interrupt driven char device I written, and it wrecked the timing of the driver.

The question I have, is why this is happening ? printk function is buffered ! it means, as far as I understand that the data is inserted in to a queue, and it's being handled later, most probably after the interrupt handler is finished.

So why doesn't it work ?

Cryan

Consider the possibility that your print-call just fills up the buffer, forcing it to flush. What will happen when doing I/O in your interrupt handler? – Huba

YES, it's really that bad. Thank you, and good night. – Scribe

It does work. printk is designed to be called from interrupt or process context. If it wasn't, it wouldn't be much use for debugging. You obviously don't call it in interrupt context in a production driver, though. – Ruminant

Answer 1

The printk function is not just inserting into a queue/buffer -- assuming the log level is high enough, the output from printk will be emitted to the console immediately, as part of the call to printk. This is especially slow if the console is, say, on a serial port. But in any case, printk does introduce pretty substantial overhead and can affect timing.

If you have a timing critical place where you want to get some debug output, you can look at using the trace_printk function in modern kernels. This actually does just put input into the trace ringbuffer, and you can read it later. Take a look at this article for full details.

Conspectus

How is debugfs as an alternative to the trace_printk? Is it good enough or does it have any caveats too? – Shanghai

That's not fully true for the kernels v3.x+ (or even starting with earlier ones) I think. OTOH the advice for trace_printk() I fully support. – Im