Re: scheduler/SCHED_FIFO behaviour

[Arun Srinivas]

hi

 I spkoe to you some days ago regarding scheduling two processes together 
on a HT.As I told you before I run them as SCHED_FIFO processes.I understood 
the theory you told me in your previous reply as to why both of SCHED_FIFO 
processes get scheduled only once and then run till completion.

But, sometimes a see a occasional reschedulei.e., the 2 processes get 
scheduled one more time after they are scheduled for the 1st time. I ran my 
code 100 times and observed this behavior 8 out of  100 times. What could be 
the reason?
(As I said i want my 2 processes to run together without any reschedule 
after they are scheduled for the first time).

Thanks
Arun
> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> To: Arun Srinivas <getarunsri@xxxxxxxxxxx>
> CC: LKML <linux-kernel@xxxxxxxxxxxxxxx>
> Subject: Re: scheduler/SCHED_FIFO behaviour
> Date: Wed, 06 Apr 2005 22:27:44 -0400
>
> On Thu, 2005-04-07 at 07:11 +0530, Arun Srinivas wrote:
> > I am not sure if my question was clear enough or I couldnt interpret you
> > answer correctly.(If it was the case I apologise for that).
> >
> > My question is, as I said I am measuring the schedule time difference
> > between my 2 of my SCHED_FIFO process in schedule() .But, I get only one 
> set
> > of readings (i.e., schedule() is being called once which implies my 
> process
> > is being scheduled only once and run till completion)
> >
> > Also, as I said my interrupts are being processed during this time.I
> > inspected /proc/interrupts for this.So, my question was if interrupts 
> heve
> > been processed several times the 2 SCHED_FIFO process which has been
> > interrupted must have been resecheduled several times and for this upon
> > returning from the interrupt handler the schedule() function must have 
> been
> > called  several times to schedule the 2 process which were running.But, 
> as I
> > said I get only one reading??
> >
> > >From your reply, I come to understand that when an interrupt interrupts 
> my
> > user process.....it runs straight way ....but upon return from the 
> interrupt
> > handler does it not call schedule() to again resume my interrupted 
> process?
>
> Exactly! Even going back to a user process, if that process is the
> highest priority process than it does not need to call schedule.
> Actually the only time it would call schedule, is if the interrupt
> called wake_up_process, or did something that needed the need_resched
> for the running task set.  Even if wake_up_process was called, if the
> process was not higher in priority than the running process, then it
> would not preempt it.
>
> So...
>
> 1) Task running in user land.
> 2) interrupt goes off, switch to kernel mode.
> 3) execute interrupt service routine.
> 4) ISR calls wake_up_process (most likely on ksoftirqd)
> 5) ksoftirqd not as high a priority as running process (don't set
> need_resched)
> 6) return from interrupt. need_resched not set.
> 7) go back to user process running in user land.
>
> There, is that clear. schedule is never called. Set ksoftirqd higher in
> priority than your tasks, and you might start seeing scheduling. But
> sometimes the functions needed to execute are done on return from
> interrupt and not though ksoftirqd, so you still might not see a
> schedule. But I'm sure you will.
>
> -- Steve

_________________________________________________________________
Trailblazer Narain Karthikeyan http://server1.msn.co.in/sp05/tataracing/ 
Will he be rookie of the year?

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