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    SubjectRe: [linux-audio-dev] Re: [announce] [patch] Voluntary Kernel Preemption Patch
    On Wed, Jul 21, 2004 at 02:30:10PM -0400, Scott Wood wrote:
    > Why is it "clearly wrong"? As far as I can tell, the only legitimate
    > use of it currently is to protect against deadlock (as in
    > spin_lock_bh()), which is not an issue if all softirqs run from a
    > thread. Ksoftirqd already ignores such disabling (unless I'm missing
    > something?), so code that uses it to synchronize with a softirq is
    > already broken.

    It appears, though, that recent kernel versions do preempt_disable()
    in ksoftirqd, apparently to support CPU hotplugging[1]. When I
    originally made the patch (against 2.6.0), this wasn't the case.
    Since it was done so recently, hopefully there are no cases since
    then that have started depending on this behavior.

    If preempt-disabled softirqs (and thus a local_bh_disable() that
    works for mutual exclusion on the local CPU) become relied upon by
    random pieces of kernel code, the ability to achieve very low
    latencies in Linux (not counting Linux bolt-ons like RT-Linux) would
    be substantially impared. At best, we'd have to use a per-CPU mutex
    to replace local_bh_disable(), causing possible performance
    degradation; this would require that any other locks held at the time
    are also turned into mutexes.


    [1] Supporting this along with non-preempt-disable softirqs would
    require some minor changes in the way that per-CPU softirqs are done
    (especially in the network code). We did these changes in our 2.4
    kernel, so as to allow priority inheritance to move a bound task
    across CPUs[2]. In particular, the softirq needs to remember which CPU
    it's processing data for, rather than using smp_processor_id() all
    over the place, and there are a few places where a per-CPU spinlock
    (only contended when the CPU binding is broken) needs to be acquired
    (all of very short duration, at least in the 2.4 code).

    Alternatively, some sort of "CPU reference count" could be used to
    withhold permission to take a CPU offline without stopping

    [2] ...which is required for priority inheritance to work properly in
    the presence of bound tasks without violating schedulability analysis
    done on other CPUs.
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