Re: [RFC] KVM: optimize the kvm_vcpu_on_spin

[Christoffer Dall]

On Sat, Jul 29, 2017 at 02:22:57PM +0800, Longpeng(Mike) wrote:
> We had disscuss the idea here:
> https://www.spinics.net/lists/kvm/msg140593.html

This is not a very nice way to start a commit description.

Please provide the necessary background to understand your change
directly in the commit message.

> 
> I think it's also suitable for other architectures.
> 

I think this sentence can go in the end of the commit message together
with your explanation of only doing this for x86.

By the way, the ARM solution should be pretty simple:

diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index a39a1e1..b9f68e4 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -416,6 +416,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
 		&& !v->arch.power_off && !v->arch.pause);
 }
 
+bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
+{
+	return vcpu_mode_priv(vcpu);
+}
+
 /* Just ensure a guest exit from a particular CPU */
 static void exit_vm_noop(void *info)
 {


I am also curious in the workload you use to measure this and how I can
evaluate the benefit on ARM?

Thanks,
-Christoffer

> If the vcpu(me) exit due to request a usermode spinlock, then
> the spinlock-holder may be preempted in usermode or kernmode.
> But if the vcpu(me) is in kernmode, then the holder must be
> preempted in kernmode, so we should choose a vcpu in kernmode
> as the most eligible candidate.
> 
> PS: I only implement X86 arch currently for I'm not familiar
>     with other architecture.
> 
> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
> ---
>  arch/mips/kvm/mips.c       | 5 +++++
>  arch/powerpc/kvm/powerpc.c | 5 +++++
>  arch/s390/kvm/kvm-s390.c   | 5 +++++
>  arch/x86/kvm/x86.c         | 5 +++++
>  include/linux/kvm_host.h   | 4 ++++
>  virt/kvm/arm/arm.c         | 5 +++++
>  virt/kvm/kvm_main.c        | 9 ++++++++-
>  7 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index d4b2ad1..2e2701d 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -98,6 +98,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>  	return !!(vcpu->arch.pending_exceptions);
>  }
>  
> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return 1;
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 1a75c0b..2489f64 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -58,6 +58,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
>  	return !!(v->arch.pending_exceptions) || kvm_request_pending(v);
>  }
>  
> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return 1;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 3f2884e..9d7c42e 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2443,6 +2443,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>  	return kvm_s390_vcpu_has_irq(vcpu, 0);
>  }
>  
> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  void kvm_s390_vcpu_block(struct kvm_vcpu *vcpu)
>  {
>  	atomic_or(PROG_BLOCK_SIE, &vcpu->arch.sie_block->prog20);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 82a63c5..b5a2e53 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8435,6 +8435,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>  	return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu);
>  }
>  
> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
> +{
> +	return kvm_x86_ops->get_cpl(vcpu) == 0;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 648b34c..f8f0d74 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -272,6 +272,9 @@ struct kvm_vcpu {
>  	} spin_loop;
>  #endif
>  	bool preempted;
> +	/* If vcpu is in kernel-mode when preempted */
> +	bool in_kernmode;
> +
>  	struct kvm_vcpu_arch arch;
>  	struct dentry *debugfs_dentry;
>  };
> @@ -797,6 +800,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  void kvm_arch_hardware_unsetup(void);
>  void kvm_arch_check_processor_compat(void *rtn);
>  int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu);
> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu);
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu);
>  
>  #ifndef __KVM_HAVE_ARCH_VM_ALLOC
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index a39a1e1..ca6a394 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -416,6 +416,11 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
>  		&& !v->arch.power_off && !v->arch.pause);
>  }
>  
> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  /* Just ensure a guest exit from a particular CPU */
>  static void exit_vm_noop(void *info)
>  {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 82987d4..8d83caa 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -290,6 +290,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>  	kvm_vcpu_set_in_spin_loop(vcpu, false);
>  	kvm_vcpu_set_dy_eligible(vcpu, false);
>  	vcpu->preempted = false;
> +	vcpu->in_kernmode = false;
>  
>  	r = kvm_arch_vcpu_init(vcpu);
>  	if (r < 0)
> @@ -2330,6 +2331,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  	int pass;
>  	int i;
>  
> +	me->in_kernmode = kvm_arch_vcpu_spin_kernmode(me);
>  	kvm_vcpu_set_in_spin_loop(me, true);
>  	/*
>  	 * We boost the priority of a VCPU that is runnable but not
> @@ -2351,6 +2353,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  				continue;
>  			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
>  				continue;
> +			if (me->in_kernmode && !vcpu->in_kernmode)
> +				continue;
>  			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
>  				continue;
>  
> @@ -4009,8 +4013,11 @@ static void kvm_sched_out(struct preempt_notifier *pn,
>  {
>  	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
>  
> -	if (current->state == TASK_RUNNING)
> +	if (current->state == TASK_RUNNING) {
>  		vcpu->preempted = true;
> +		vcpu->in_kernmode = kvm_arch_vcpu_spin_kernmode(vcpu);
> +	}
> +
>  	kvm_arch_vcpu_put(vcpu);
>  }
>  
> -- 
> 1.8.3.1
> 
>