> On Fri, 25 Feb 2022 at 23:04, Li RongQing <lirongqing@xxxxxxxxx> wrote: > > > > When sending a call-function IPI-many to vCPUs, yield to the IPI > > target vCPU which is marked as preempted. > > > > but when emulating HLT, an idling vCPU will be voluntarily scheduled > > out and mark as preempted from the guest kernel perspective. yielding > > to idle vCPU is pointless and increase unnecessary vmexit, maybe miss > > the true preempted vCPU > > > > so yield to IPI target vCPU only if vCPU is busy and preempted > > This is not correct, there is an intention to boost the reactivation of idle vCPU, > PV sched yield is used in over-subscribe scenario and the pCPU which idle vCPU is > resident maybe busy, and the vCPU will wait in the host scheduler run queue. > There is a research paper [1] focusing on this boost and showing better > performance numbers, though their boost is more unfair. > > [1]. https://ieeexplore.ieee.org/document/8526900 > "Accelerating Idle vCPU Reactivation" > > Wanpeng I understand that over-subscribe system is not always over- subscribe, it should sometime true, sometime not. Without this patch, it will hard the performance when cpu is not over-subscribe. And yielding to a not ready vcpu is unnecessary as kvm_sched_yield static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id) { if (!target || !READ_ONCE(target->ready)) goto no_yield; } -Li
