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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 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


spinlock desc->lock
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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