[PATCH v3] KVM: X86: Just one leader to trigger kvmclock sync request

[PATCH v3] KVM: X86: Just one leader to trigger kvmclock sync request

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

In the progress of vCPUs creation, it queues a kvmclock sync worker to the global
workqueue before each vCPU creation completes. The workqueue subsystem guarantees 
not to queue the already queued work, however, we can make the logic more clear by 
make just one leader to trigger this kvmclock sync request and save on cacheline 
boucing due to test_and_set_bit.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
v2 -> v3:
 * update patch description
v1 -> v2:
 * check vcpu->vcpu_idx

 arch/x86/kvm/x86.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fb5d64e..79bc995 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9390,8 +9390,9 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 	if (!kvmclock_periodic_sync)
 		return;
 
-	schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
-					KVMCLOCK_SYNC_PERIOD);
+	if (vcpu->vcpu_idx == 0)
+		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
+						KVMCLOCK_SYNC_PERIOD);
 }
 
 void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
-- 
2.7.4

Re: [PATCH v3] KVM: X86: Just one leader to trigger kvmclock sync request

[Vitaly Kuznetsov]

Wanpeng Li <kernellwp@gmail.com> writes:

> From: Wanpeng Li <wanpengli@tencent.com>
>
> In the progress of vCPUs creation, it queues a kvmclock sync worker to the global
> workqueue before each vCPU creation completes. The workqueue subsystem guarantees 
> not to queue the already queued work, however, we can make the logic more clear by 
> make just one leader to trigger this kvmclock sync request and save on cacheline 
> boucing due to test_and_set_bit.
>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v2 -> v3:
>  * update patch description
> v1 -> v2:
>  * check vcpu->vcpu_idx
>
>  arch/x86/kvm/x86.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fb5d64e..79bc995 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9390,8 +9390,9 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
>  	if (!kvmclock_periodic_sync)
>  		return;
>  
> -	schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> -					KVMCLOCK_SYNC_PERIOD);
> +	if (vcpu->vcpu_idx == 0)
> +		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> +						KVMCLOCK_SYNC_PERIOD);

I would've merged this new check with !kvmclock_periodic_sync above
making it more obvious when the work is scheduled

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5de200663f51..93550976f991 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9389,11 +9389,9 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 
        mutex_unlock(&vcpu->mutex);
 
-       if (!kvmclock_periodic_sync)
-               return;
-
-       schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
-                                       KVMCLOCK_SYNC_PERIOD);
+       if (vcpu->vcpu_idx == 0 && kvmclock_periodic_sync)
+               schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
+                                     KVMCLOCK_SYNC_PERIOD);
 }

>  }
>  
>  void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)

With or without the change mentioned above,
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly

Re: [PATCH v3] KVM: X86: Just one leader to trigger kvmclock sync request

[Paolo Bonzini]

On 02/03/20 14:01, Vitaly Kuznetsov wrote:
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9389,11 +9389,9 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
>  
>         mutex_unlock(&vcpu->mutex);
>  
> -       if (!kvmclock_periodic_sync)
> -               return;
> -
> -       schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> -                                       KVMCLOCK_SYNC_PERIOD);
> +       if (vcpu->vcpu_idx == 0 && kvmclock_periodic_sync)
> +               schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> +                                     KVMCLOCK_SYNC_PERIOD);
>  }

> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Good idea, I squashed the change.

Paolo