Re: [PATCH 3/3] KVM: arm64: Disabling disabled PMU counters wastes a lot of time

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

On 7/13/21 2:59 PM, Marc Zyngier wrote:
> From: Alexandre Chartre <alexandre.chartre@oracle.com>
>
> In a KVM guest on arm64, performance counters interrupts have an
> unnecessary overhead which slows down execution when using the "perf
> record" command and limits the "perf record" sampling period.
>
> The problem is that when a guest VM disables counters by clearing the
> PMCR_EL0.E bit (bit 0), KVM will disable all counters defined in
> PMCR_EL0 even if they are not enabled in PMCNTENSET_EL0.
>
> KVM disables a counter by calling into the perf framework, in particular
> by calling perf_event_create_kernel_counter() which is a time consuming
> operation. So, for example, with a Neoverse N1 CPU core which has 6 event
> counters and one cycle counter, KVM will always disable all 7 counters
> even if only one is enabled.
>
> This typically happens when using the "perf record" command in a guest
> VM: perf will disable all event counters with PMCNTENTSET_EL0 and only
> uses the cycle counter. And when using the "perf record" -F option with
> a high profiling frequency, the overhead of KVM disabling all counters
> instead of one on every counter interrupt becomes very noticeable.
>
> The problem is fixed by having KVM disable only counters which are
> enabled in PMCNTENSET_EL0. If a counter is not enabled in PMCNTENSET_EL0
> then KVM will not enable it when setting PMCR_EL0.E and it will remain
> disabled as long as it is not enabled in PMCNTENSET_EL0. So there is
> effectively no need to disable a counter when clearing PMCR_EL0.E if it
> is not enabled PMCNTENSET_EL0.
>
> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> [maz: moved 'mask' close to the actual user, simplifying the patch]
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Link: https://lore.kernel.org/r/20210712170345.660272-1-alexandre.chartre@oracle.com
> ---
>  arch/arm64/kvm/pmu-emul.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index fae4e95b586c..dc65b58dc68f 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -563,20 +563,21 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val)
>   */
>  void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
>  {
> -	unsigned long mask = kvm_pmu_valid_counter_mask(vcpu);
>  	int i;
>  
>  	if (val & ARMV8_PMU_PMCR_E) {
>  		kvm_pmu_enable_counter_mask(vcpu,
>  		       __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
>  	} else {
> -		kvm_pmu_disable_counter_mask(vcpu, mask);
> +		kvm_pmu_disable_counter_mask(vcpu,
> +		       __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
>  	}
>  
>  	if (val & ARMV8_PMU_PMCR_C)
>  		kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX, 0);
>  
>  	if (val & ARMV8_PMU_PMCR_P) {
> +		unsigned long mask = kvm_pmu_valid_counter_mask(vcpu);
>  		mask &= ~BIT(ARMV8_PMU_CYCLE_IDX);
>  		for_each_set_bit(i, &mask, 32)
>  			kvm_pmu_set_counter_value(vcpu, i, 0);

Looks reasonable to me, and it fixes the issue described in the commit:

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

Thanks,

Alex

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