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

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