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SubjectRe: [RFC] Scheduler issue 1, RT tasks ...
Davide Libenzi wrote:
> I'd like to have some comments about RT tasks implementation in a SMP
> system because POSIX it's not clear about how the priority rules apply to
> multiprocessor systems.
> The Balanced Multi Queue Scheduler ( BMQS, )
> i'm working on tries to keep CPU schedulers the more independent as
> possible and currently implements two kind of RT tasks, local one and
> global ones.
> Local RT tasks apply POSIX priority rules inside the local CPU, that means
> that an RT task running on CPU0 cannot preempt another task ( being it
> normal or RT ) on CPU1. This keeps schedulers interlocking very low
> because of the very fast path in reschedule_idle() ( no multi lock
> acquisition, CPU queue loops, etc...).
> Global RT tasks, that live in a separate run queue, have the ability to
> preempt remote CPU and this can lead ( in the unfortunate case that the
> last CPU running the RT task is running another RT task ) to an higher
> cost in reschedule_idle().
> The check for a global RT task selection is done in a very fast way before
> checking the local queue :
> if (!list_empty(&runqueue_head(RT_QID)))
> goto rt_queue_select;
> rt_queue_select_back:
> and this does not affect the scheduler latency at all.
> On the contrary, by having a separate queue for global RT tasks, can
> improve it in high run queue load cases.
> The local/global RT task selection is done with setscheduler() with a new
> ( or'ed ) flag SCHED_RTGLOBAL, and this means that the default is RT task
> local.
> I'd like to have comments on this before jumping to the next Scheduler
> issue ( balancing mode ).
My understanding of the POSIX standard is the the highest priority
task(s) are to get the cpu(s) using the standard calls. If you want to
deviate from this I think the standard allows extensions, but they IMHO
should be requested, not the default, so I would turn your flag around
to force LOCAL, not GLOBAL.

Real time sched:
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