linux-kernel - Re: [PATCH v4 1/2] KVM: X86: Less kvmclock sync induced vmexits after VM boots

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Date:   Tue, 18 Feb 2020 16:33:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v4 1/2] KVM: X86: Less kvmclock sync induced vmexits after
 VM boots

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?