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[PATCH v5] KVM: halt-polling: poll for the upcoming fire timers

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[PATCH v5] KVM: halt-polling: poll for the upcoming fire timers

Wanpeng Li-4
From: Wanpeng Li <[hidden email]>

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.

echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests in
order to use high-res preemption tick. So task switch will trigger by
hrtimer fire in guests. halt_poll_ns_timer is set to 10000ns.

Context switching - times in microseconds - smaller is better
-------------------------------------------------------------------------
Host                 OS  2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
                         ctxsw  ctxsw  ctxsw ctxsw  ctxsw   ctxsw   ctxsw
--------- ------------- ------ ------ ------ ------ ------ ------- -------
kernel     Linux 4.6.0+ 7.9800   11.0   10.8   14.6 9.4300    13.0    10.2 vanilla
kernel     Linux 4.6.0+   15.3   13.6   10.7   12.5 9.0000    12.8 7.38000 poll

Cc: Paolo Bonzini <[hidden email]>
Cc: Radim Krčmář <[hidden email]>
Cc: David Matlack <[hidden email]>
Cc: Christian Borntraeger <[hidden email]>
Cc: Yang Zhang <[hidden email]>
Signed-off-by: Wanpeng Li <[hidden email]>
---
v4 -> v5:
 * add module parameter halt_poll_ns_timer to enable/disable this feature
v3 -> v4:
 * add module parameter halt_poll_ns_timer
 * rename patch subject since lapic maybe just for x86.
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                 | 13 ++++++++++---
 10 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index 0df6b1f..fdfbed9 100644
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -292,6 +292,10 @@ 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 e63d23b..f510d71 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -371,6 +371,10 @@ 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 6733ac5..baf9472 100644
--- a/arch/mips/include/asm/kvm_host.h
+++ b/arch/mips/include/asm/kvm_host.h
@@ -814,6 +814,10 @@ 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 c805cf4..1b89a68 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..4c1914a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -78,6 +78,10 @@ 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_timer = 0;
+module_param(halt_poll_ns_timer, uint, S_IRUGO | S_IWUSR);
+
 /*
  * Ordering of locks:
  *
@@ -2014,12 +2018,15 @@ 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_timer) {
+ 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

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Re: [PATCH v5] KVM: halt-polling: poll for the upcoming fire timers

Paolo Bonzini


On 25/05/2016 04:47, Wanpeng Li wrote:
> From: Wanpeng Li <[hidden email]>
>
> 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.

As discussed on IRC, I would like to understand why the adaptive
adjustment of halt_poll_ns is failing.  It seems like you have so few
halts that you don't get halt_poll_ns>0.  Yet, when the VM halts, it's
very close to the timer tick---often enough for this patch to have an
effect.

Please send a trace of halt_poll_ns_grow and halt_poll_ns_shrink
tracepoints, so that we can find out more about this.

Thanks,

Paolo

> 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.
>
> echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests in
> order to use high-res preemption tick. So task switch will trigger by
> hrtimer fire in guests. halt_poll_ns_timer is set to 10000ns.
>
> Context switching - times in microseconds - smaller is better
> -------------------------------------------------------------------------
> Host                 OS  2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
>                          ctxsw  ctxsw  ctxsw ctxsw  ctxsw   ctxsw   ctxsw
> --------- ------------- ------ ------ ------ ------ ------ ------- -------
> kernel     Linux 4.6.0+ 7.9800   11.0   10.8   14.6 9.4300    13.0    10.2 vanilla
> kernel     Linux 4.6.0+   15.3   13.6   10.7   12.5 9.0000    12.8 7.38000 poll
>
> Cc: Paolo Bonzini <[hidden email]>
> Cc: Radim Krčmář <[hidden email]>
> Cc: David Matlack <[hidden email]>
> Cc: Christian Borntraeger <[hidden email]>
> Cc: Yang Zhang <[hidden email]>
> Signed-off-by: Wanpeng Li <[hidden email]>
> ---
> v4 -> v5:
>  * add module parameter halt_poll_ns_timer to enable/disable this feature
> v3 -> v4:
>  * add module parameter halt_poll_ns_timer
>  * rename patch subject since lapic maybe just for x86.
> 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                 | 13 ++++++++++---
>  10 files changed, 48 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 0df6b1f..fdfbed9 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -292,6 +292,10 @@ 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 e63d23b..f510d71 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -371,6 +371,10 @@ 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 6733ac5..baf9472 100644
> --- a/arch/mips/include/asm/kvm_host.h
> +++ b/arch/mips/include/asm/kvm_host.h
> @@ -814,6 +814,10 @@ 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 c805cf4..1b89a68 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..4c1914a 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -78,6 +78,10 @@ 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_timer = 0;
> +module_param(halt_poll_ns_timer, uint, S_IRUGO | S_IWUSR);
> +
>  /*
>   * Ordering of locks:
>   *
> @@ -2014,12 +2018,15 @@ 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_timer) {
> + 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 {
>   /*
>
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Re: [PATCH v5] KVM: halt-polling: poll for the upcoming fire timers

Paolo Bonzini


On 26/05/2016 12:26, Paolo Bonzini wrote:
> As discussed on IRC, I would like to understand why the adaptive
> adjustment of halt_poll_ns is failing.  It seems like you have so few
> halts that you don't get halt_poll_ns>0.  Yet, when the VM halts, it's
> very close to the timer tick---often enough for this patch to have an
> effect.
>
> Please send a trace of halt_poll_ns_grow and halt_poll_ns_shrink
> tracepoints, so that we can find out more about this.

And 30 seconds after I wrote this email, you told me on IRC that the
guest had HZ=1000 and the module parameter was set to 1 ms in order to
_really_ benefit from the patch.  So basically you could obtain the same
effect with idle=poll in the guest.

This explains why your reported results were not so great (as David noted).

Thanks,

Paolo
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Re: [PATCH v5] KVM: halt-polling: poll for the upcoming fire timers

Wanpeng Li-4
2016-05-26 18:30 GMT+08:00 Paolo Bonzini <[hidden email]>:

>
>
> On 26/05/2016 12:26, Paolo Bonzini wrote:
>> As discussed on IRC, I would like to understand why the adaptive
>> adjustment of halt_poll_ns is failing.  It seems like you have so few
>> halts that you don't get halt_poll_ns>0.  Yet, when the VM halts, it's
>> very close to the timer tick---often enough for this patch to have an
>> effect.
>>
>> Please send a trace of halt_poll_ns_grow and halt_poll_ns_shrink
>> tracepoints, so that we can find out more about this.
>
> And 30 seconds after I wrote this email, you told me on IRC that the
> guest had HZ=1000 and the module parameter was set to 1 ms in order to
> _really_ benefit from the patch.  So basically you could obtain the same
> effect with idle=poll in the guest.
>
> This explains why your reported results were not so great (as David noted).

Yeah, I will drop the patch.

Regards,
Wanpeng Li
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