2016-05-20 10:04 GMT+08:00 Yang Zhang <yang.zhang.wz@xxxxxxxxx>: > On 2016/5/20 2:36, David Matlack wrote: >> >> 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.) > > > This feature is slightly different from current advance expiration way. > Advance expiration rely on the VCPU is running(do polling before vmentry). > But in some cases, the timer interrupt may be blocked by other thread(i.e., > IF bit is clear) and VCPU cannot be scheduled to run immediately. So even > advance the timer early, VCPU may still see the latency. But polling is > different, it ensures the VCPU to aware the timer expiration before schedule > out. Great explanation, Yang! I prefer to include this statement in my patch description. > >> >>> 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? > > > It should be reasonable. I have seen the same improvement with ctx switch > benchmark: The latency is reduce from ~2600ns to ~2300ns with the similar > mechanism.(The same idea but different implementation) Good to know it. ;-) Regards, Wanpeng Li > >>> >>>> >>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> >>>> Cc: Radim Krčmář <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 >>>> >> -- >> To unsubscribe from this list: send the line "unsubscribe kvm" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > > -- > best regards > yang -- Regards, Wanpeng Li -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
