Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

From: David Matlack
Date: Thu May 19 2016 - 14:37:17 EST

On Thu, May 19, 2016 at 11:01 AM, David Matlack <dmatlack@xxxxxxxxxx> wrote:
> On Thu, May 19, 2016 at 6:27 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> If an emulated lapic timer will fire soon(in the scope of 10us the
>> base of dynamic halt-polling, lower-end of message passing workload
>> latency TCP_RR's poll time < 10us) we can treat it as a short halt,
>> and poll to wait it fire, the fire callback apic_timer_fn() will set
>> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
>> This can avoid context switch overhead and the latency which we wake
>> up vCPU.
>
> If I understand correctly, your patch aims to reduce the latency of
> (APIC Timer expires) -> (Guest resumes execution) using halt-polling.
> Let me know if I'm misunderstanding.
>
> In general, I don't think it makes sense to poll for timer interrupts.
> We know when the timer interrupt is going to arrive. If we care about
> the latency of delivering that interrupt to the guest, we should
> program the hrtimer to wake us up slightly early, and then deliver the
> virtual timer interrupt right on time (I think KVM's TSC Deadline
> Timer emulation already does this).

(It looks like the way to enable this feature is to set the module
parameter lapic_timer_advance_ns and make sure your guest is using the
TSC Deadline timer instead of the APIC Timer.)

> I'm curious to know if this scheme
> would give the same performance improvement to iperf as your patch.
>
> We discussed this a bit before on the mailing list before
> (https://lkml.org/lkml/2016/3/29/680). I'd like to see halt-polling
> and timer interrupts go in the opposite direction: if the next timer
> event (from any timer) is less than vcpu->halt_poll_ns, don't poll at
> all.
>
>>
>> iperf TCP get ~6% bandwidth improvement.
>
> Can you explain why your patch results in this bandwidth improvement?
>
>>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Cc: David Matlack <dmatlack@xxxxxxxxxx>
>> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>> v1 -> v2:
>> * add return statement to non-x86 archs
>> * capture never expire case for x86 (hrtimer is not started)
>>
>> arch/arm/include/asm/kvm_host.h | 4 ++++
>> arch/arm64/include/asm/kvm_host.h | 4 ++++
>> arch/mips/include/asm/kvm_host.h | 4 ++++
>> arch/powerpc/include/asm/kvm_host.h | 4 ++++
>> arch/s390/include/asm/kvm_host.h | 4 ++++
>> arch/x86/kvm/lapic.c | 11 +++++++++++
>> arch/x86/kvm/lapic.h | 1 +
>> arch/x86/kvm/x86.c | 5 +++++
>> include/linux/kvm_host.h | 1 +
>> virt/kvm/kvm_main.c | 14 ++++++++++----
>> 10 files changed, 48 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index 4cd8732..a5fd858 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -284,6 +284,10 @@ static inline void kvm_arch_sync_events(struct kvm *kvm) {}
>> static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>> static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
>> +static inline u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu)
>> +{
>> + return -1ULL;
>> +}
>>
>> static inline void kvm_arm_init_debug(void) {}
>> static inline void kvm_arm_setup_debug(struct kvm_vcpu *vcpu) {}
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index d49399d..94e227a 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -359,6 +359,10 @@ static inline void kvm_arch_sync_events(struct kvm *kvm) {}
>> static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>> static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
>> +static inline u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu)
>> +{
>> + return -1ULL;
>> +}
>>
>> void kvm_arm_init_debug(void);
>> void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
>> diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
>> index 9a37a10..456bc42 100644
>> --- a/arch/mips/include/asm/kvm_host.h
>> +++ b/arch/mips/include/asm/kvm_host.h
>> @@ -813,6 +813,10 @@ static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>> static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}
>> +static inline u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu)
>> +{
>> + return -1ULL;
>> +}
>> static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
>>
>> #endif /* __MIPS_KVM_HOST_H__ */
>> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
>> index ec35af3..5986c79 100644
>> --- a/arch/powerpc/include/asm/kvm_host.h
>> +++ b/arch/powerpc/include/asm/kvm_host.h
>> @@ -729,5 +729,9 @@ static inline void kvm_arch_exit(void) {}
>> static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
>> +static inline u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu)
>> +{
>> + return -1ULL;
>> +}
>>
>> #endif /* __POWERPC_KVM_HOST_H__ */
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index 37b9017..bdb01a1 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -696,6 +696,10 @@ static inline void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>> struct kvm_memory_slot *slot) {}
>> static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu) {}
>> static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}
>> +static inline u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu)
>> +{
>> + return -1ULL;
>> +}
>>
>> void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu);
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index bbb5b28..cfeeac3 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -256,6 +256,17 @@ static inline int apic_lvtt_tscdeadline(struct kvm_lapic *apic)
>> return apic->lapic_timer.timer_mode == APIC_LVT_TIMER_TSCDEADLINE;
>> }
>>
>> +u64 apic_get_timer_expire(struct kvm_vcpu *vcpu)
>> +{
>> + struct kvm_lapic *apic = vcpu->arch.apic;
>> + struct hrtimer *timer = &apic->lapic_timer.timer;
>> +
>> + if (!hrtimer_active(timer))
>> + return -1ULL;
>> + else
>> + return ktime_to_ns(hrtimer_get_remaining(timer));
>> +}
>> +
>> static inline int apic_lvt_nmi_mode(u32 lvt_val)
>> {
>> return (lvt_val & (APIC_MODE_MASK | APIC_LVT_MASKED)) == APIC_DM_NMI;
>> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
>> index 891c6da..ee4da6c 100644
>> --- a/arch/x86/kvm/lapic.h
>> +++ b/arch/x86/kvm/lapic.h
>> @@ -212,4 +212,5 @@ bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
>> struct kvm_vcpu **dest_vcpu);
>> int kvm_vector_to_index(u32 vector, u32 dest_vcpus,
>> const unsigned long *bitmap, u32 bitmap_size);
>> +u64 apic_get_timer_expire(struct kvm_vcpu *vcpu);
>> #endif
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index a8c7ca3..9b5ad99 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7623,6 +7623,11 @@ bool kvm_vcpu_compatible(struct kvm_vcpu *vcpu)
>> struct static_key kvm_no_apic_vcpu __read_mostly;
>> EXPORT_SYMBOL_GPL(kvm_no_apic_vcpu);
>>
>> +u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu)
>> +{
>> + return apic_get_timer_expire(vcpu);
>> +}
>> +
>> int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>> {
>> struct page *page;
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index b1fa8f1..14d6c23 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -663,6 +663,7 @@ int kvm_vcpu_yield_to(struct kvm_vcpu *target);
>> void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu);
>> void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
>> void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
>> +u64 kvm_arch_timer_remaining(struct kvm_vcpu *vcpu);
>>
>> void kvm_flush_remote_tlbs(struct kvm *kvm);
>> void kvm_reload_remote_mmus(struct kvm *kvm);
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index dd4ac9d..e4bb30b 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -78,6 +78,9 @@ module_param(halt_poll_ns_grow, uint, S_IRUGO | S_IWUSR);
>> static unsigned int halt_poll_ns_shrink;
>> module_param(halt_poll_ns_shrink, uint, S_IRUGO | S_IWUSR);
>>
>> +/* lower-end of message passing workload latency TCP_RR's poll time < 10us */
>> +static unsigned int halt_poll_ns_base = 10000;
>> +
>> /*
>> * Ordering of locks:
>> *
>> @@ -1966,7 +1969,7 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
>> grow = READ_ONCE(halt_poll_ns_grow);
>> /* 10us base */
>> if (val == 0 && grow)
>> - val = 10000;
>> + val = halt_poll_ns_base;
>> else
>> val *= grow;
>>
>> @@ -2014,12 +2017,15 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> ktime_t start, cur;
>> DECLARE_SWAITQUEUE(wait);
>> bool waited = false;
>> - u64 block_ns;
>> + u64 block_ns, delta, remaining;
>>
>> + remaining = kvm_arch_timer_remaining(vcpu);
>> start = cur = ktime_get();
>> - if (vcpu->halt_poll_ns) {
>> - ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
>> + if (vcpu->halt_poll_ns || remaining < halt_poll_ns_base) {
>> + ktime_t stop;
>>
>> + delta = vcpu->halt_poll_ns ? vcpu->halt_poll_ns : remaining;
>> + stop = ktime_add_ns(ktime_get(), delta);
>> ++vcpu->stat.halt_attempted_poll;
>> do {
>> /*
>> --
>> 1.9.1
>>