[PATCH v2] KVM: arm64: Disabling disabled PMU counters wastes a lot of time
Robin Murphy
robin.murphy at arm.com
Mon Jul 12 08:44:06 PDT 2021
On 2021-07-12 16:17, Alexandre Chartre wrote:
> 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 at oracle.com>
> ---
> The patch is based on https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pmu/reset-values
>
> arch/arm64/kvm/pmu-emul.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index fae4e95b586c..1f317c3dac61 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -563,21 +563,23 @@ 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);
> + unsigned long mask;
> 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) {
> - mask &= ~BIT(ARMV8_PMU_CYCLE_IDX);
> + mask = kvm_pmu_valid_counter_mask(vcpu)
> + & BIT(ARMV8_PMU_CYCLE_IDX);
This looks suspiciously opposite of what it replaces; however did we
even need to do a bitwise operation here in the first place? Couldn't we
skip the cycle counter by just limiting the for_each_set_bit iteration
below to 31 bits?
Robin.
> for_each_set_bit(i, &mask, 32)
> kvm_pmu_set_counter_value(vcpu, i, 0);
> }
>
> base-commit: 83f870a663592797c576846db3611e0a1664eda2
>
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