Re: scheduler/SCHED_FIFO behaviour

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 2005-04-05 at 04:36 +0530, Arun Srinivas wrote:
> I am scheduling 2 SCHED_FIFO processes and set them affinity( process A runs 
> on processor 1 and process B runs on processor 2), on a HT processor.(I did 
> this cause I wanted to run them together).Now, in schedule() I measure the 
> timedifference between when they are scheduled. I found that when I 
> introduce these 2 processes as SCHED_FIFO they are
> 
> 1)scheduled only once and run till completion ( they running time is around 
> 2 mins.)

If they are the highest priority task, and running as FIFO this is the
proper behavior.

> 2)entire system appears frozen....no mouse/key presses detected until the 
> processes exit.
> 

If X is not at a higher priority than the test you are running, it will
never get a chance to run.

> >From what I observed does it mean that even the OS / interrupt handler does 
> not occur during the entire period of time these real time processes run?? 
> (as I said the processes run in minutes).

The interrupts do get processed. Now the bottom halves and tasklets may
be starved if they are set at a lower priority than your test (ie. the
ksoftirqd thread). But most likely they are processed too.

> How can I verify that?
> 

#!/bin/sh
cat /proc/interrupts
run_test
cat /proc/interrupts

If the run_test takes 2 minutes, you should see a large difference in
the two outputs.

-- Steve

> Thanks
> Arun