Messages in this thread | | | Date | Tue, 13 Sep 2011 17:51:58 +0300 | From | Avi Kivity <> | Subject | Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops |
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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
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