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SubjectRe: Real Time Runqueue
On Fri, Nov 16, 2001 at 04:28:33PM -0800, Davide Libenzi wrote:
> On Fri, 16 Nov 2001, Mike Kravetz wrote:
> > 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.
> Well I don't know how RT sematics apply to SMP systems.

Me either (not exactly).

> The easy solution ( == big common lock ) would be to have a single RT
> queue that is checked before the private one.
> Anyway, sometime it happens that the cure is worst than the disease and to
> solve a corner case you're going to punish common case performances (
> Linux is not an RT OS even with that fix ).

The reason I ask is that we went through the pains of a separate
realtime RQ in our MQ scheduler. And yes, it does hurt the common
case, not to mention the extra/complex code paths. I was hoping
that someone in the know could enlighten us as to how RT semantics
apply to SMP systems. If the semantics I suggest above are required,
then it implies support must be added to any possible future
scheduler implementations.

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