Re: [libvirt] [PATCH 0/7] x86: Rework KVM-defaults compat code, enable kvm_pv_unhalt by default

On Wed, Oct 11, 2017 at 04:19:38PM -0400, Waiman Long wrote: > On 10/10/2017 03:41 PM, Eduardo Habkost wrote: >> On Tue, Oct 10, 2017 at 02:07:25PM -0400, Waiman Long wrote: >>> On 10/10/2017 11:50 AM, Eduardo Habkost wrote: >>>>> Yes. Another possibility is to enable it when there is >1 NUMA node in >>>>> the guest. We generally don't do this kind of magic but higher layers >>>>> (oVirt/OpenStack) do. >>>> Can't the guest make this decision, instead of the host? >>> By guest, do you mean the guest OS itself or the admin of the guest VM? >> It could be either. But even if action is required from the >> guest admin to get better performance in some cases, I'd argue >> that the default behavior of a Linux guest shouldn't cause a >> performance regression if the host stops hiding a feature in >> CPUID. >> >>> I am thinking about maybe adding kernel boot command line option like >>> "unfair_pvspinlock_cpu_threshold=4" which will instruct the OS to use >>> unfair spinlock if the number of CPUs is 4 or less, for example. The >>> default value of 0 will have the same behavior as it is today. Please >>> let me know what you guys think about that. >> If that's implemented, can't Linux choose a reasonable default >> for unfair_pvspinlock_cpu_threshold that won't require the admin >> to manually configure it on most cases? > It is hard to have a fixed value as it depends on the CPUs being used as > well as the kind of workloads that are being run. Besides, using unfair > locks have the undesirable side effect of being subject to lock > starvation under certain circumstances. So we may not work it to be > turned on by default. Customers have to take their own risk if they want > that. Probably I am not seeing all variables involved, so pardon my confusion. Would unfair_pvspinlock_cpu_threshold > num_cpus just disable usage of kvm_pv_unhalt, or make the guest choose a completely different spinlock implementation?

Is the current default behavior of Linux guests when kvm_pv_unhalt is unavailable a good default? If using kvm_pv_unhalt is not always a good idea, why do Linux guests default to eagerly trying to use it only because the host says it's available?