linux-kernel - Re: [PATCH v2] KVM: x86/pmu: Reduce counter period change overhead and delay the effective time

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Date:   Wed, 8 Apr 2020 22:04:34 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     pbonzini@...hat.com
Cc:     ehankland@...gle.com, jmattson@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, wanpengli@...cent.com
Subject: Re: [PATCH v2] KVM: x86/pmu: Reduce counter period change overhead
 and delay the effective time

Hi Paolo,

Could you please take a look at this patch?
If there is anything needs to be improved, please let me know.

Thanks,
Like Xu

On 2020/3/26 20:47, Like Xu wrote:
> Anyone to help review this change?
> 
> Thanks,
> Like Xu
> 
> On 2020/3/17 16:14, Like Xu wrote:
>> The cost of perf_event_period() is unstable, and when the guest samples
>> multiple events, the overhead increases dramatically (5378 ns on E5-2699).
>>
>> For a non-running counter, the effective time of the new period is when
>> its corresponding enable bit is enabled. Calling perf_event_period()
>> in advance is superfluous. For a running counter, it's safe to delay the
>> effective time until the KVM_REQ_PMU event is handled. If there are
>> multiple perf_event_period() calls before handling KVM_REQ_PMU,
>> it helps to reduce the total cost.
>>
>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>> ---
>>   arch/x86/kvm/pmu.c           | 11 -----------
>>   arch/x86/kvm/pmu.h           | 11 +++++++++++
>>   arch/x86/kvm/vmx/pmu_intel.c | 10 ++++------
>>   3 files changed, 15 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index d1f8ca57d354..527a8bb85080 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -437,17 +437,6 @@ void kvm_pmu_init(struct kvm_vcpu *vcpu)
>>       kvm_pmu_refresh(vcpu);
>>   }
>> -static inline bool pmc_speculative_in_use(struct kvm_pmc *pmc)
>> -{
>> -    struct kvm_pmu *pmu = pmc_to_pmu(pmc);
>> -
>> -    if (pmc_is_fixed(pmc))
>> -        return fixed_ctrl_field(pmu->fixed_ctr_ctrl,
>> -            pmc->idx - INTEL_PMC_IDX_FIXED) & 0x3;
>> -
>> -    return pmc->eventsel & ARCH_PERFMON_EVENTSEL_ENABLE;
>> -}
>> -
>>   /* Release perf_events for vPMCs that have been unused for a full time 
>> slice.  */
>>   void kvm_pmu_cleanup(struct kvm_vcpu *vcpu)
>>   {
>> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
>> index d7da2b9e0755..cd112e825d2c 100644
>> --- a/arch/x86/kvm/pmu.h
>> +++ b/arch/x86/kvm/pmu.h
>> @@ -138,6 +138,17 @@ static inline u64 get_sample_period(struct kvm_pmc 
>> *pmc, u64 counter_value)
>>       return sample_period;
>>   }
>> +static inline bool pmc_speculative_in_use(struct kvm_pmc *pmc)
>> +{
>> +    struct kvm_pmu *pmu = pmc_to_pmu(pmc);
>> +
>> +    if (pmc_is_fixed(pmc))
>> +        return fixed_ctrl_field(pmu->fixed_ctr_ctrl,
>> +            pmc->idx - INTEL_PMC_IDX_FIXED) & 0x3;
>> +
>> +    return pmc->eventsel & ARCH_PERFMON_EVENTSEL_ENABLE;
>> +}
>> +
>>   void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel);
>>   void reprogram_fixed_counter(struct kvm_pmc *pmc, u8 ctrl, int fixed_idx);
>>   void reprogram_counter(struct kvm_pmu *pmu, int pmc_idx);
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index 7c857737b438..20f654a0c09b 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -263,15 +263,13 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, 
>> struct msr_data *msr_info)
>>               if (!msr_info->host_initiated)
>>                   data = (s64)(s32)data;
>>               pmc->counter += data - pmc_read_counter(pmc);
>> -            if (pmc->perf_event)
>> -                perf_event_period(pmc->perf_event,
>> -                          get_sample_period(pmc, data));
>> +            if (pmc_speculative_in_use(pmc))
>> +                kvm_make_request(KVM_REQ_PMU, vcpu);
>>               return 0;
>>           } else if ((pmc = get_fixed_pmc(pmu, msr))) {
>>               pmc->counter += data - pmc_read_counter(pmc);
>> -            if (pmc->perf_event)
>> -                perf_event_period(pmc->perf_event,
>> -                          get_sample_period(pmc, data));
>> +            if (pmc_speculative_in_use(pmc))
>> +                kvm_make_request(KVM_REQ_PMU, vcpu);
>>               return 0;
>>           } else if ((pmc = get_gp_pmc(pmu, msr, MSR_P6_EVNTSEL0))) {
>>               if (data == pmc->eventsel)
>>
>