[PATCH 1/2] ARM: KVM: Yield CPU when vcpu executes a WFE

[Raghavendra KT]

On Mon, Oct 7, 2013 at 9:10 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> 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 at arm.com>
> ---
>  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);

Could you also enable CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT for arm and
check if ple handler logic helps further?
we would ideally get one more optimization folded into ple handler if
you enable that.