On 09/27/2012 12:08 PM, Gleb Natapov wrote: > On Thu, Sep 27, 2012 at 12:04:58PM +0200, Avi Kivity wrote: >> On 09/27/2012 11:58 AM, Gleb Natapov wrote: >> > >> >> > >> >> >> btw, we can have secondary effects. A vcpu can be waiting for a lock in >> >> >> the host kernel, or for a host page fault. There's no point in boosting >> >> >> anything for that. Or a vcpu in userspace can be waiting for a lock >> >> >> that is held by another thread, which has been preempted. >> >> > Do you mean userspace spinlock? Because otherwise task that's waits on >> >> > a kernel lock will sleep in the kernel. >> >> >> >> I meant a kernel mutex. >> >> >> >> vcpu 0: take guest spinlock >> >> vcpu 0: vmexit >> >> vcpu 0: spin_lock(some_lock) >> >> vcpu 1: take same guest spinlock >> >> vcpu 1: PLE vmexit >> >> vcpu 1: wtf? >> >> >> >> Waiting on a host kernel spinlock is not too bad because we expect to be >> >> out shortly. Waiting on a host kernel mutex can be a lot worse. >> >> >> > We can't do much about it without PV spinlock since there is not >> > information about what vcpu holds which guest spinlock, no? >> >> It doesn't help. If the lock holder is waiting for another lock in the >> host kernel, boosting it doesn't help even if we know who it is. We >> need to boost the real lock holder, but we have no idea who it is (and >> even if we did, we often can't do anything about it). >> > Without PV lock we will boost random preempted vcpu instead of going to > sleep in the situation you described. True. In theory boosting a random vcpu shouldn't have any negative effects though. Right now the problem is that the boosting itself is expensive. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
