Re: [RFC] KVM: optimize the kvm_vcpu_on_spin

[Longpeng (Mike)]

On 2017/7/31 21:22, Christoffer Dall wrote:

> 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.
> 


OK :)

> 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?
> 


We had tested this using the SpecVirt testsuite, no improvement (no decrease at
least) because of the spinlock isn't the major factor of this testsuite.

Currently I haven't any performance numbers to prove the patch is make sense,
but I'll do some tests later.

> 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
>>
>>
> 
> .
> 


-- 
Regards,
Longpeng(Mike)