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

From: [email-masked] (Marc Zyngier)
Date: Tue, 08 Oct 2013 13:43:09 +0100

On 08/10/13 12:26, Raghavendra KT wrote:
> On Mon, Oct 7, 2013 at 9:10 PM, Marc Zyngier <[email-masked]> 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 <[email-masked]>
>> ---
>>  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.

Just gave it a go, and the results are slightly (but consistently)
worse. Over 10 runs:

Without RELAX_INTERCEPT: Average run 3.3623s
With RELAX_INTERCEPT: Average run 3.4226s

Not massive, but still noticeable. Any clue?

	M.
-- 
Jazz is not dead. It just smells funny...