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SubjectRe: Real Time Runqueue
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


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.

--------- ---------
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.

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