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    SubjectRe: [RFC][PATCH RT 0/2] futex priority based wakeup

    * Sébastien Dugué <sebastien.dugue@bull.net> wrote:

    > in the current futex implementation, tasks are woken up in FIFO
    > order, (i.e. in the order they were put to sleep). For realtime
    > systems needing system wide strict realtime priority scheduling, tasks
    > should be woken up in priority order.
    >
    > This patchset achieves this by changing the futex hash bucket list
    > into a plist. Tasks waiting on a futex are enqueued in this plist
    > based on their priority so that they can be woken up in priority
    > order.

    hm, i dont think this is enough. Basically, waking up in priority order
    is just the (easier) half of the story - what you want is to also
    propagate priorities when you block. We provided a complete solution via
    the PI-futex patchset (currently included in -mm).

    In other words: as long as locking primitives go, i dont think real-time
    applications should use wakeup-priority-ordered futexes, they should use
    the real thing, PI futexes.

    There is one exception: when a normal futex is used as a waitqueue
    without any contention properties. (for example a waitqueue for worker
    threads) But those are both rare, and typically dont muster tasks with
    different priorities - i.e. FIFO is good enough.

    Also, there's a performance cost to this. Could you try to measure the
    impact to SCHED_OTHER tasks via some pthread locking benchmark?

    Ingo
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