| Message ID | 20210712151700.654819-1-alexandre.chartre@oracle.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] KVM: arm64: Disabling disabled PMU counters wastes a lot of time | expand |
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@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 >
Hi Robin, On 7/12/21 4:44 PM, Robin Murphy wrote: > 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@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; It always sets the bit, which goes against the architecture and the code it was replacing, yes. > 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? To quote myself [1]: "Entertained the idea of restricting the number of bits in for_each_set_bit() to 31 since Linux (and the architecture, to some degree) treats the cycle count register as the 32nd event counter. Settled on this approach because I think it's clearer." To expand on that, incorrectly resetting the cycle counter was introduced by a refactoring, so I preferred making it very clear that PMCR_EL0.P is not supposed to clear the cycle counter. [1] https://lore.kernel.org/kvmarm/20210618105139.83795-1-alexandru.elisei@arm.com/ Thanks, Alex > > Robin. > >> for_each_set_bit(i, &mask, 32) >> kvm_pmu_set_counter_value(vcpu, i, 0); >> } >> >> base-commit: 83f870a663592797c576846db3611e0a1664eda2 >>
On 7/12/21 5:51 PM, Alexandru Elisei wrote: > Hi Robin, > > On 7/12/21 4:44 PM, Robin Murphy wrote: >> 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@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; > > It always sets the bit, which goes against the architecture and the code it was > replacing, yes. > My bad, I screw up and I dropped the ~. I will resend. Sorry, alex. >> 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? > > To quote myself [1]: > > "Entertained the idea of restricting the number of bits in for_each_set_bit() to > 31 since Linux (and the architecture, to some degree) treats the cycle count > register as the 32nd event counter. Settled on this approach because I think it's > clearer." > > To expand on that, incorrectly resetting the cycle counter was introduced by a > refactoring, so I preferred making it very clear that PMCR_EL0.P is not supposed > to clear the cycle counter. > > [1] https://lore.kernel.org/kvmarm/20210618105139.83795-1-alexandru.elisei@arm.com/ > > Thanks, > > Alex > >> >> Robin. >> >>> for_each_set_bit(i, &mask, 32) >>> kvm_pmu_set_counter_value(vcpu, i, 0); >>> } >>> >>> base-commit: 83f870a663592797c576846db3611e0a1664eda2 >>>
On 2021-07-12 16:51, Alexandru Elisei wrote: > Hi Robin, > > On 7/12/21 4:44 PM, Robin Murphy wrote: >> 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@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; > > It always sets the bit, which goes against the architecture and the code it was > replacing, yes. > >> 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? > > To quote myself [1]: > > "Entertained the idea of restricting the number of bits in for_each_set_bit() to > 31 since Linux (and the architecture, to some degree) treats the cycle count > register as the 32nd event counter. FWIW I wouldn't say there's any degree to it - we're iterating over the bits in a register where the cycle counter enable is unequivocally the 32nd bit. > Settled on this approach because I think it's > clearer." > > To expand on that, incorrectly resetting the cycle counter was introduced by a > refactoring, so I preferred making it very clear that PMCR_EL0.P is not supposed > to clear the cycle counter. Fair enough, but if this has turned out to be a contentious hot path then masking the bit to zero and then deliberately iterating to see if it's set (find_next_bit() isn't exactly free) adds up to more overhead than a comment ;) Robin. > > [1] https://lore.kernel.org/kvmarm/20210618105139.83795-1-alexandru.elisei@arm.com/ > > Thanks, > > Alex > >> >> Robin. >> >>> for_each_set_bit(i, &mask, 32) >>> kvm_pmu_set_counter_value(vcpu, i, 0); >>> } >>> >>> base-commit: 83f870a663592797c576846db3611e0a1664eda2 >>>
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); for_each_set_bit(i, &mask, 32) kvm_pmu_set_counter_value(vcpu, i, 0); }
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> --- 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(-) base-commit: 83f870a663592797c576846db3611e0a1664eda2