Re: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers
From: David Matlack
Date: Tue May 24 2016 - 18:39:11 EST
On Tue, May 24, 2016 at 12:57 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.
>
> 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.
>
> 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
These results aren't very compelling. Sometimes polling is faster,
sometimes vanilla is faster, sometimes they are about the same.
I imagine there are hyper sensitive workloads which cannot tolerate a
long tail in timer latency (e.g. realtime workloads). I would expect a
patch like this to provide a "smoothing effect", reducing that tail.
But for cloud/server workloads, I would not expect any sensitivity to
jitter in timer latency (especially while the VCPU is halted).
Note that while halt-polling happens when the CPU is idle, it's still
not free. It constricts the scheduler's cpu load balancer, because the
CPU appears to be busy. In KVM's default configuration, I'd prefer to
only add more polling when the gain is clear. If there are guest
workloads that want this patch, I'd suggest polling for timers be
default-off. At minimum, there should be a module parameter to control
it (like Christian Borntraeger suggested).
>
> 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>
> ---
> 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 | 15 +++++++++++----
> 10 files changed, 49 insertions(+), 4 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..afd15ba 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 = 10000;
> +module_param(halt_poll_ns_timer, uint, S_IRUGO | S_IWUSR);
> +
> /*
> * Ordering of locks:
> *
> @@ -1966,7 +1970,7 @@ 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_timer;
> else
> val *= grow;
>
> @@ -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
>