Re: [Lse-tech] Re: Real Time Runqueue

[Andi Kleen]

On Mon, Nov 19, 2001 at 10:32:23AM -0800, george anzinger wrote:
> Andi Kleen wrote:
> > 
> > On Fri, Nov 16, 2001 at 04:32:24PM -0800, Mike Kravetz wrote:
> > > 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.
> > 
> > It seems a lot of applications/APIs do not care about global RT semantics,
> > but about RT semantics for groups of threads or processes (e.g. java
> > or ada applications). Linux currently simulates this only for root
> > and with a global runqueue. 
> 
> Why do you say only root?  Since the schedule type and priority are

You effectively need to be root to set up the priorities at least once.
setuid programs are a big pain to use and a lot of people don't want them.

Also it is useful to change priorities on the fly, e.g. to avoid
priority inheritance problems. 

> > I don't think it makes too much sense to have
> > an global rt queue on a multi processor system, but there should be some
> > way to define "scheduling groups" where rt semantics are followed inside.
> 
> Still, the customer is king.

Hmm?

> 
> > Such a scheduling group could be a clone flag or default to CLONE_VM for
> > example for compatibility.  A scheduling group would also make it possible
> > to support simple rt semantics for thread groups as non root.  Then one
> > could run a rt queue per scheduling group, and simulate global rt run queue
> > or per cpu rt run queue as needed by appropiate setup.
> 
> My first thought is that this is fairly high overhead to put in the
> schedule path.  May be if I knew more....

The only overhead as far as I can see would be a few more pointer
references for RT processes (task->runqueue-> ...). I guess with
some care it can be even kept out of the non RT fast path. 

-Andi