Re: RT task scheduling

[Darren Hart <darren@dvhart.com>]

On Thursday 06 April 2006 00:37, Ingo Molnar wrote:
> * Darren Hart <darren@dvhart.com> wrote:
> > My last mail specifically addresses preempt-rt, but I'd like to know
> > people's thoughts regarding this issue in the mainline kernel.  Please
> > see my previous post "realtime-preempt scheduling - rt_overload
> > behavior" for a testcase that produces unpredictable scheduling
> > results.
>
> the rt_overload feature i intend to push upstream-wards too, i just
> didnt separate it out of -rt yet.

Great news!

>
> "RT overload scheduling" is a totally orthogonal mechanism to the SMP
> load-balancer (and this includes smpnice too) that is more or less
> equivalent to having a 'global runqueue' for real-time tasks, without
> the SMP overhead associated with that. If there is no "RT overload" [the
> common case even on Linux systems that _do_ make use of RT tasks
> occasionally], the new mechanism is totally inactive and there's no
> overhead. 

Agreed.  smpnice is geared toward load_balancing (which indicates an imbalance 
already exists).  In order to achieve "system wide strict realtime priority 
scheduling" we need to avoid that priority imbalance altogether.

> But once there are more RT tasks than CPUs, the scheduler will
> do "global" decisions for what RT tasks to run on which CPU. To put even
> less overhead on the mainstream kernel, i plan to introduce a new
> SCHED_FIFO_GLOBAL scheduling policy to trigger this behavior. [it doesnt
> make much sense to extend SCHED_RR in that direction.]

I agree that SCHED_RR doesn't need to be included here.  I'm not sure about 
another scheduling policy though.  As you said, the existing mechanism is 
inactive with nr_rt_tasks <= NR_CPUS, applications using more than that (with 
SCHED_FIFO) will likely want the rt_overload feature - as you said, it's 
about predictability and determinism.  As it is now we are using POSIX 
standard scheduling policies - do we want to add a non standard one?  I don't 
see the benefit.

> my gut feeling is that it would be wrong to integrate this feature into
> smpnice: SCHED_FIFO is about determinism, and smpnice is a fundamentally
> statistical approach. Also, smpnice doesnt have to try as hard to pick
> the right task as rt_overload does, so there would be constant
> 'friction' between "overhead" optimizations (dont be over-eager) and
> "latency" optimizations (dont be _under_-eager). So i'm quite sure we
> want this feature separate. [nevertheless i'd happy to be proven wrong
> via some good and working smpnice based solution]
>
> in any case, i'll check your -rt testcase to see why it fails.

Just so I am clear, is the goal then to achieve "system wide strict realtime 
priority scheduling", as opposed to a best effort?  I think this makes the 
most sense and it seems to me that the rt_overload mechanism is intended for 
just that.


Thanks,

--Darren
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