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    SubjectRe: RT task scheduling
    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.
    >
    > "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. But once there are more RT tasks than CPUs, the scheduler will
    > do "global" decisions for what RT tasks to run on which CPU.

    Is this good enough? Isn't it possible with the current
    try_to_wake_up() implementation (with or without smpnice) for two RT
    tasks to end up on the same CPU even when there are less RT tasks than CPUs?

    > 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 wouldn't have thought a new policy was necessary. Why not just do
    this for all SCHED_FIFO (or even all RT) tasks?

    BTW Does any body in the real world actually use SCHED_RR?

    Peter
    --
    Peter Williams pwil3058@bigpond.net.au

    "Learning, n. The kind of ignorance distinguishing the studious."
    -- Ambrose Bierce
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