Re: [PATCH v3] KVM: halt-polling: poll if emulated lapic timer will fire soon

[David Matlack]

On Sun, May 22, 2016 at 5:42 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

I'm ok with this patch, but I'd like to better understand the target
workloads. What type of workloads do you expect to benefit from this?

>
> 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.
>
> 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.
>
> iperf TCP get ~6% bandwidth improvement.

I think my question got lost in the previous thread :). Can you
explain why TCP bandwidth improves with this patch?

>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v2 -> v3:
>  * add Yang's statement to patch description
> 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));

There's a chance for a time-of-check to time-of-use race here, where
the timer expires after checking if it's active. It seems benign though,
I'd expect hrtimer_get_remaining() to underflow, which is the desired
behavior. In the worse case we halt-poll for up to 10 us, which is ok.
Do you agree?

> +}
> +
>  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
>