[PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

[PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

[Waiman Long]

On a VM with only 1 vCPU, the locking fast path will always be
successful. In this case, there is no need to use the the PV qspinlock
code which has higher overhead on the unlock side than the native
qspinlock code.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/x86/kernel/kvm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5b2300b..575c9a5 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -748,6 +748,10 @@ void __init kvm_spinlock_init(void)
 	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
 		return;
 
+	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
+	if (num_possible_cpus() == 1)
+		return;
+
 	__pv_init_lock_hash();
 	pv_lock_ops.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
 	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
-- 
1.8.3.1

Re: [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

[Paolo Bonzini]

On 17/07/2018 23:59, Waiman Long wrote:
> On a VM with only 1 vCPU, the locking fast path will always be
> successful. In this case, there is no need to use the the PV qspinlock
> code which has higher overhead on the unlock side than the native
> qspinlock code.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/x86/kernel/kvm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 5b2300b..575c9a5 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -748,6 +748,10 @@ void __init kvm_spinlock_init(void)
>  	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
>  		return;
>  
> +	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
> +	if (num_possible_cpus() == 1)
> +		return;
> +
>  	__pv_init_lock_hash();
>  	pv_lock_ops.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>  	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
> 

Queued, thanks.

Paolo

Re: [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

[Konrad Rzeszutek Wilk]

On Tue, Jul 17, 2018 at 05:59:27PM -0400, Waiman Long wrote:
> On a VM with only 1 vCPU, the locking fast path will always be
> successful. In this case, there is no need to use the the PV qspinlock
> code which has higher overhead on the unlock side than the native
> qspinlock code.

Why not make this global? That is for both KVM and Xen and any
other virtualized guest that uses this?

> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/x86/kernel/kvm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 5b2300b..575c9a5 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -748,6 +748,10 @@ void __init kvm_spinlock_init(void)
>  	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
>  		return;
>  
> +	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
> +	if (num_possible_cpus() == 1)
> +		return;
> +
>  	__pv_init_lock_hash();
>  	pv_lock_ops.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>  	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
> -- 
> 1.8.3.1
> 

Re: [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

[Waiman Long]

On 07/18/2018 09:15 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 17, 2018 at 05:59:27PM -0400, Waiman Long wrote:
>> On a VM with only 1 vCPU, the locking fast path will always be
>> successful. In this case, there is no need to use the the PV qspinlock
>> code which has higher overhead on the unlock side than the native
>> qspinlock code.
> Why not make this global? That is for both KVM and Xen and any
> other virtualized guest that uses this?

Right, I will send another patch for Xen. The pvqspinlock code has to be
explicitly opted in. Right now, both Xen and KVM used it in the tree. I
am not sure about other out-of-tree modules. There is nothing I can do
for those.

Cheers,
Longman