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SubjectRe: [PATCH RFC 4/4] xen: implement Xen-specific spinlocks
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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