linux-kernel - Re: [PATCH v3 1/2] KVM: X86: Less kvmclock sync induced vmexits after VM boots

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Date:   Mon, 17 Feb 2020 18:40:19 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 1/2] KVM: X86: Less kvmclock sync induced vmexits after
 VM boots

On Mon, 17 Feb 2020 at 18:36, Wanpeng Li <kernellwp@...il.com> wrote:
>
> From: Wanpeng Li <wanpengli@...cent.com>
>
> In the progress of vCPUs creation, it queues a kvmclock sync worker to
> the global
> workqueue before each vCPU creation completes. Each worker will be scheduled
> after 300 * HZ delay and request a kvmclock update for all vCPUs and kick them
> out. This is especially worse when scaling to large VMs due to a lot of vmexits.
> Just one worker as a leader to trigger the kvmclock sync request for
> all vCPUs is
> enough.

Sorry for the alignment.

>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  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..d0ba2d4 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 (kvm->created_vcpus == 1)
> +        schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> +                        KVMCLOCK_SYNC_PERIOD);
>  }
>
>  void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
> --
> 2.7.4