Messages in this thread | | | Date | Tue, 08 Jul 2008 00:15:21 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH RFC 4/4] xen: implement Xen-specific spinlocks |
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Johannes Weiner wrote: >> +static DEFINE_PER_CPU(int, lock_kicker_irq) = -1; >> +static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners); >> > > The plural is a bit misleading, as this is a single pointer per CPU. >
Yeah. And it's wrong because it's specifically *not* spinning, but blocking.
>> +static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl) >> +{ >> + int cpu; >> + >> + for_each_online_cpu(cpu) { >> > > Would it be feasible to have a bitmap for the spinning CPUs in order to > do a for_each_spinning_cpu() here instead? Or is setting a bit in > spinning_lock() and unsetting it in unspinning_lock() more overhead than > going over all CPUs here? >
Not worthwhile, I think. This is a very rare path: it will only happen if 1) there's lock contention, that 2) wasn't resolved within the timeout. In practice, this gets called a few thousand times per cpu over a kernbench, which is nothing.
My very original version of this code kept a bitmask of interested CPUs within the lock, but there's only space for 24 cpus if we still use a byte for the lock itself. It all turned out fairly awkward, and this version is a marked improvement.
J
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