Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v4 1/2] KVM: X86: Less kvmclock sync induced vmexits after
 VM boots
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
References: <1581988630-19182-1-git-send-email-wanpengli@tencent.com>
 <87r1ys7xpk.fsf@vitty.brq.redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <e6caee13-f8f7-596c-fb37-6120e7c25f99@redhat.com>
Date:   Tue, 18 Feb 2020 16:33:44 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.1.1
MIME-Version: 1.0
In-Reply-To: <87r1ys7xpk.fsf@vitty.brq.redhat.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 18/02/20 15:54, Vitaly Kuznetsov wrote:
>> -	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)
> Forgive me my ignorance, I was under the impression
> schedule_delayed_work() doesn't do anything if the work is already
> queued (see queue_delayed_work_on()) and we seem to be scheduling the
> same work (&kvm->arch.kvmclock_sync_work) which is per-kvm (not
> per-vcpu).

No, it executes after 5 minutes.  I agree that the patch shouldn't be
really necessary, though you do save on cacheline bouncing due to
test_and_set_bit.

Paolo

> Do we actually happen to finish executing it before next vCPU
> is created or why does the storm you describe happens?