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    SubjectRe: Real-Time Preemption and RCU

    * Paul E. McKenney <> wrote:

    > 4. Preemptible read side.
    > RCU read-side critical sections can (in theory, anyway) be quite
    > large, degrading realtime scheduling response. Preemptible RCU
    > read-side critical sections avoid such degradation. Manual
    > insertion of "preemption points" might be an alternative as well.
    > But I have no intention of trying to settle the long-running
    > argument between proponents of preemption and of preemption
    > points. Not today, anyway! ;-)

    i'm cleverly sidestepping that argument by offering 4 preemption models
    in the -RT tree :-) We dont have to pick a winner, users will. The way
    low latencies are achieved depends on the preemption model:

    ( ) No Forced Preemption (Server)
    ( ) Voluntary Kernel Preemption (Desktop)
    ( ) Preemptible Kernel (Low-Latency Desktop)
    (X) Complete Preemption (Real-Time)

    "Server" is the current default !PREEMPT model. Best throughput, bad
    worst-case latencies.

    "Low-Latency Desktop" is the current PREEMPT model. Has some runtime
    overhead relative to Server, offers fair worst-case latencies.

    "Desktop" is a new mode that is somewhere between Server and Low-Latency
    Desktop: it's what was initially called 'voluntary preemption'. It
    doesnt make the kernel forcibly preemptible anywhere, but inserts a fair
    number of preemption points to decrease latencies statistically, while
    keeping the runtime overhead close to the "Server". (a variant of this
    model is utilized by Fedora and RHEL4 currently, so if this gets
    upstream i expect distros to pick it up - it can be a migration helper
    towards the "Low-Latency Desktop" model.)

    "Real-Time" is the no-compromises hard-RT model: almost everything but
    the scheduler itself is fully preemptible. Phenomenal worst-case
    latencies in every workload scenario, but also has the highest runtime

    preemptable RCU makes sense for the "Low-Latency Desktop" and
    "Real-Time" preemption models - these are the ones that do forced

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