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    SubjectRe: new IRQ scalability changes in 2.3.48
    On Fri, Mar 03, 2000 at 03:25:23PM +0100, Ingo Molnar wrote:
    >
    > On Fri, 3 Mar 2000 yodaiken@chelm.cs.nmt.edu wrote:
    >
    > > On Sun, Feb 27, 2000 at 04:04:13PM +0100, Ingo Molnar wrote:
    > > > - per-IRQ-source spinlocks and per-IRQ-controller spinlocks
    > > > increasing scalability: now two IRQ handlers on two CPUs
    > > > can run do_IRQ in parallel. Note that level-triggered PCI IRQ
    > > > handlers never actually take the IRQ-controller spinlock in the
    > > > 'IRQ handling fast path'.
    > >
    > > This change puts spinlock back into the low level irq code -- and I took
    > > them out specifically to make RTLinux work and to make Linus happy since
    > > he was concerned about minimizing the number of spinlocks. The
    > > "optimization" means that you can, in parallel run a tiny section of code
    > > that is called infrequently -- [...]
    >
    > it actually makes a huge difference as the _real_ optimization you missed
    > is the lack of cacheline ping-pongs, which pingpong inevitably happens if
    > globally shared spinlocks are used.

    As in the spin lock in do_IRQ which is still there and now duplicated.
    As far as I can see, x86 now does

    interrupt:

    do_IRQ
    spinlock desc->lock
    handler->ack
    spinlock again

    The old code had a spinlock per controller, allowing for
    parallel operation. The new code seems to simply duplicate
    spin locks.


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