Re: Real Time Runqueue

[Mike Kravetz]

On Fri, Nov 16, 2001 at 02:44:30PM -0800, Davide Libenzi wrote:
> 
> I do not use a separate queue coz, if it's single, it becomes a common
> lock for all CPUs.
> RT tasks are scheduled as usual and the only problem arises in
> reschedule_idle() when an RT task is pushed onto the run queue when
> 1) on its CPU it is _not_ running the idle
> 2) on its CPU is running another RT task with higher priority
> 
> In that case a "good CPU" discovery loop is triggered, the task is moved
> on that CPU runqueue, need_resched is set, an IPI is sent and on return
> from the remote CPU IPI  path the RT task is run.
> A good solution would be ( i'm not doing it now ), in setscheduler() to
> move the task in a way to have an even distribution of RT tasks among
> CPUs.
> 
> - Davide

Davide,

Suppose you have a 2 CPU system with 4 runnable tasks.  3 of these
tasks are realtime with the same realtime priority and the other is
an ordinary SCHED_OTHER task.  The task distribution on the runqueues
looks something like this.

CPU 0             CPU 1
---------         ---------
RT Task A         RT Task B
Other Task C      RT Task D

Task A and Task B are currently running on the 2 CPUs.  Now, Task A
voluntarily gives up CPU 0 and Task B is still running on CPU 1.
At this point, Task D should be chosen to run on CPU 0.  Correct?
Isn't this a required RT semantic?  I'm curious how you plan on
accomplishing this.

Regards,
-- 
Mike