On Sat, Jun 25, 2016 at 11:21:30PM +0800, Boqun Feng wrote:
> >
> > int vpc = vcpu_preempt_count();
> >
> > ...
> >
> > for (;;) {
> >
> > /* the big spin loop */
> >
> > if (need_resched() || vpc != vcpu_preempt_count())
>
> So on PPC, we have lppaca::yield_count to detect when an vcpu is
Which sounds like just the value we want.. And I suspect that on x86 KVM
and Xen have similar numbers stashed away someplace.
> preempted, if the yield_count is even, the vcpu is running, otherwise it
> is preempted(__spin_yield() is a user of this).
>
> Therefore it makes more sense we
>
> if (need_resched() || vcpu_is_preempted(old))
>
> here, and implement vcpu_is_preempted() on PPC as
>
> bool vcpu_is_preempted(int cpu)
> {
> return !!(be32_to_cpu(lppaca_of(cpu).yield_count) & 1)
> }
>
> Thoughts?
That works here, but it would not work for the need_resched() in
mutex_spin_on_owner() and mutex_optimistic_spin() which need equal
treatment.
Because those too we want to limit.
The count thing, while a little more cumbersome, is more widely
applicable than just the one OSQ case where we happen to have a cpu
number.
