On an (even slightly) oversubscribed system, spinlocks are quickly becoming a bottleneck, as some vcpus are spinning, waiting for a lock to be released, while the vcpu holding the lock may not be running at all. This creates contention, and the observed slowdown is 40x for hackbench. No, this isn't a typo. The solution is to trap blocking WFEs and tell KVM that we're now spinning. This ensures that other vpus will get a scheduling boost, allowing the lock to be released more quickly. >From a performance point of view: hackbench 1 process 1000 2xA15 host (baseline): 1.843s 2xA15 guest w/o patch: 2.083s 4xA15 guest w/o patch: 80.212s 2xA15 guest w/ patch: 2.072s 4xA15 guest w/ patch: 3.202s So we go from a 40x degradation to 1.5x, which is vaguely more acceptable. Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> --- arch/arm/include/asm/kvm_arm.h | 4 +++- arch/arm/kvm/handle_exit.c | 6 +++++- 2 files changed, 8 insertions(+), 2 deletions(-) diff --git a/arch/arm/include/asm/kvm_arm.h b/arch/arm/include/asm/kvm_arm.h index 64e9696..693d5b2 100644 --- a/arch/arm/include/asm/kvm_arm.h +++ b/arch/arm/include/asm/kvm_arm.h @@ -67,7 +67,7 @@ */ #define HCR_GUEST_MASK (HCR_TSC | HCR_TSW | HCR_TWI | HCR_VM | HCR_BSU_IS | \ HCR_FB | HCR_TAC | HCR_AMO | HCR_IMO | HCR_FMO | \ - HCR_SWIO | HCR_TIDCP) + HCR_TWE | HCR_SWIO | HCR_TIDCP) #define HCR_VIRT_EXCP_MASK (HCR_VA | HCR_VI | HCR_VF) /* System Control Register (SCTLR) bits */ @@ -208,6 +208,8 @@ #define HSR_EC_DABT (0x24) #define HSR_EC_DABT_HYP (0x25) +#define HSR_WFI_IS_WFE (1U << 0) + #define HSR_HVC_IMM_MASK ((1UL << 16) - 1) #define HSR_DABT_S1PTW (1U << 7) diff --git a/arch/arm/kvm/handle_exit.c b/arch/arm/kvm/handle_exit.c index df4c82d..c4c496f 100644 --- a/arch/arm/kvm/handle_exit.c +++ b/arch/arm/kvm/handle_exit.c @@ -84,7 +84,11 @@ static int handle_dabt_hyp(struct kvm_vcpu *vcpu, struct kvm_run *run) static int kvm_handle_wfi(struct kvm_vcpu *vcpu, struct kvm_run *run) { trace_kvm_wfi(*vcpu_pc(vcpu)); - kvm_vcpu_block(vcpu); + if (kvm_vcpu_get_hsr(vcpu) & HSR_WFI_IS_WFE) + kvm_vcpu_on_spin(vcpu); + else + kvm_vcpu_block(vcpu); + return 1; } -- 1.8.2.3 -- 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