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    SubjectRe: [linux-audio-dev] Re: [announce] [patch] Voluntary Kernel Preemption Patch

    * Andrew Morton <> wrote:

    > None of these approaches improves worst-case latency at all on SMP.
    > If we're not going to address the SMP problem we could just make it
    > UP-only, in which case increased locking costs are a non-issue.
    > I'd prefer that we find a solution for SMP too though.

    i have solved the fundamental SMP latency problems in the -Q7 patch, by
    redesigning how SMP preemption is done. Here's the relevant changelog


    the main change in this patch are more SMP latency fixes. The stock
    kernel, even with CONFIG_PREEMPT enabled, didnt have any spin-nicely
    preemption logic for the following, commonly used SMP locking
    primitives: read_lock(), spin_lock_irqsave(), spin_lock_irq(),
    spin_lock_bh(), read_lock_irqsave(), read_lock_irq(), read_lock_bh(),
    write_lock_irqsave(), write_lock_irq(), write_lock_bh(). Only
    spin_lock() and write_lock() [the two simplest cases] where covered.

    In addition to the preemption latency problems, the _irq() variants in
    the above list didnt do any IRQ-enabling while spinning - possibly
    resulting in excessive irqs-off sections of code!

    -Q7 fixes all of these latency problems: we now re-enable interrupts
    while spinning in all possible cases, and a spinning op stays
    preemptible if this is a beginning of a new critical section.


    feedback from Mark H Johnson:

    the latest patch is:

    i'm already in the process of cleaning up the patch and making it ready
    for splitup & merge. The spinlock fixes will be amongst the first
    patches i'll send you.

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