Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

Avi Kivity
2011-09-13T14:52:14Z

On 09/13/2011 05:22 PM, Peter Zijlstra wrote:
> On Tue, 2011-09-13 at 14:43 +0300, Avi Kivity wrote:
>
> > Another issue is that hypervisors use PAUSE to detect a spinning guest
> > and issue a directed yield to another vcpu. But for cmpxchg loops, the
> > "spinner" would just commit on the next loop, no? So I think there's no
> > objection from that front.
>
> Right, we shouldn't ever spend a significant amount spinning on a
> cmpxchg. If we do we need to fix that instead.

I hate arguing while agreeing, but the issue isn't that we don't spend a
significant time spinning, but that there is no owner. Should the other
cpu go away, we just pick up a new copy of oldval and complete the
transaction.

With spinlocks, even if you hold it for just a single guest cycle, the
situation is different. If the vcpu that holds the spinlock is
preempted, the spinner is forced to spin until the owner is rescheduled.

--
error compiling committee.c: too many arguments to function