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    SubjectRe: Real-Time Preemption and RCU

    * Herbert Xu <herbert@gondor.apana.org.au> wrote:

    > Ingo Molnar <mingo@elte.hu> wrote:
    > >
    > > i really have no intention to allow multiple readers for rt-mutexes. We
    > > got away with that so far, and i'd like to keep it so. Imagine 100
    > > threads all blocked in the same critical section (holding the read-lock)
    > > when a highprio writer thread comes around: instant 100x latency to let
    > > all of them roll forward. The only sane solution is to not allow
    > > excessive concurrency. (That limits SMP scalability, but there's no
    > > other choice i can see.)
    >
    > What about allowing only as many concurrent readers as there are CPUs?

    since a reader may be preempted by a higher prio task, there is no
    linear relationship between CPU utilization and the number of readers
    allowed. You could easily end up having all the nr_cpus readers
    preempted on one CPU. It gets pretty messy.

    Ingo
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