Re: sched /HT processor

[Arun Srinivas]

Thanks. yes, a reschedule may not take place after a ms, if the currently 
running task cannot be preempted by another task.

(1) But, can a reschedule happen within a millisec (or once a process is 
scheduled can schedule() be called before the next millisec.) ?

2) Also in case argument (1) is not true, and I want rescheduling to be done 
(i.e., schedule() called) in less than 1 ms , can I directly change the HZ 
value in <asm-i386/param.h> and recompile my kernel so that my timer 
interrupt will occur frequently?

Thanks
Arun

> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> To: Jesper Juhl <juhl-lkml@xxxxxx>
> CC: Arun Srinivas <getarunsri@xxxxxxxxxxx>,        LKML 
> <linux-kernel@xxxxxxxxxxxxxxx>
> Subject: Re: sched /HT processor
> Date: Sun, 03 Apr 2005 11:31:03 -0400
>
> On Sun, 2005-04-03 at 13:17 +0200, Jesper Juhl wrote:
> >
> > A reschedule can happen once every ms, but also upon returning to
> > userspace and when returning from an interrupt handler, and also when
> > something in the kernel explicitly calls schedule() or sleeps (which in
> > turn results in a call to schedule()). And each CPU runs schedule()
> > independently.
> > At least that's my understanding of it - if I'm wrong I hope someone on
> > the list will correct me.
>
> You're correct, but I'll add some more details here.  The actual
> schedule happens when needed.  A schedule may not take place at every
> ms, if the task running is not done with its time slice and no events
> happened where another task should preempt it. If an RT task is running
> in a FIFO policy, then it will continue to run until it calls schedule
> itself or another process of higher priority preempts it.
>
> Now if you don't have PREEMPT turned on, than the schedule won't take
> place at all while a task is in the kernel, unless the task explicitly
> calls schedule.
>
> What happens on a timer interrupt where a task is done with its time
> slice or another event where a schedule should take place, is just the
> need_resched flag is set for the task.  On return from the interrupt the
> flag is checked, and if set a schedule is called.
>
> This is still a pretty basic description of what really happens, and if
> you want to learn more, just start searching the kernel code for
> schedule and need_resched. Don't forget to look in the asm code (ie
> entry.S, and dependent on your arch other *.S files).
>
> -- Steve

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