RE: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

David Laight
2021-03-30T08:32:32Z

From: Guo Ren
> Sent: 30 March 2021 04:14
...
> > Step 1 would be to get your architecute fixed such that it can provide
> > fwd progress guarantees for LL/SC. Otherwise there's absolutely no point
> > in building complex systems with it.
>
> Quote Waiman's comment [1] on xchg16 optimization:
>
> "This optimization is needed to make the qspinlock achieve performance
> parity with ticket spinlock at light load."
>
> [1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email-Waiman.Long@hp.com/
>
> So for a non-xhg16 machine:
> - ticket-lock for small numbers of CPUs
> - qspinlock for large numbers of CPUs
>
> Okay, I'll put all of them into the next patch :P

Doesn't that also imply that you need ticket-locks for
lightly contended locks even with a lot of CPUs?

If you actually get a lot of CPUs waiting on the same spinlock
you probably ought to change the code to reduce lock contention.

David

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