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

Hi,

Jeremy Fitzhardinge [off-list ref] writes:

Johannes Weiner wrote:

quoted

quoted

+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.

I thought of it as `virtually spinning', so had no problems with the
naming itself :)

quoted

quoted

+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.

Okay, I agree.

	Hannes