interrupt latency

Hi All, I used the Linux kernel’s ‘Ftrace’ framework for tracing the interrupt latencies. Sometimes I noticed a delay(time between do_IRQ() to start of ISR()) in start of ISR(interrupt service routine) to execute , some other times I noticed ISR execution time(entering and exiting a function) is very high. I’d like to know in which scenarios, ISR gets executed lately? In which scenarios, ISR execution time is very high? Can someone please clarify me on this. -- Thanks, Sekhar