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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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