isr - finding longest interrupt in linux -

i have system have interrupt taking long time (several ms), , causing real-time issues. problem don't know interrupt is. i'm using ftrace, irqsoff, , it's reporting longest irq-off period starts @ handle_int , ends @ __do_softirq (hence, it's in interrupt). there easy way determine interrupt causing delay? (note: kernel version 3.4)

here example manual of trace-cmd-record:

trace-cmd record -p function_graph -e irq_handler_entry  -l do_irq sleep 10