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

On 10/13/2017 03:01 PM, Eduardo Habkost wrote: > 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? What I am proposing is that if num_cpus <= unfair_pvspinlock_cpu_threshold, the unfair spinlock will be used even if kvm_pv_unhalt is set. > 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? For kernel with CONFIG_PARVIRT_SPINLOCKS, the current default is to use pvqspinlock if kvm_pv_unhalt is enabled, but use unfair spinlock if it is disabled. For kernel with just CONFIG_PARVIRT but no CONFIG_PARAVIRT_SPINLOCKS, the unfair lock will be use no matter the setting of kvm_pv_unhalt. Without those config options, the standard qspinlock will be used.