Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon
From: Yang Zhang
Date: Thu May 19 2016 - 22:04:19 EST
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.
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)
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
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best regards
yang