linux-kernel - Re: [PATCH RESEND] KVM: Boosting vCPUs that are delivering interrupts

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Message-ID: <f6f9c2ca-6fea-d7b5-9797-d180e42f50d5@redhat.com>
Date:   Thu, 18 Jul 2019 11:39:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH RESEND] KVM: Boosting vCPUs that are delivering interrupts

On 18/07/19 11:29, Wanpeng Li wrote:
> On Thu, 18 Jul 2019 at 17:07, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> On 18/07/19 10:43, Wanpeng Li wrote:
>>>>> Isnt that done by the sched_in handler?
>>>>
>>>> I am a bit confused because, if it is done by the sched_in later, I
>>>> don't understand why the sched_out handler hasn't set vcpu->preempted
>>>> already.
>>>>
>>>> The s390 commit message is not very clear, but it talks about "a former
>>>> sleeping cpu" that "gave up the cpu voluntarily".  Does "voluntarily"
>>>> that mean it is in kvm_vcpu_block?  But then at least for x86 it would
>>>
>>> see the prepare_to_swait_exlusive() in kvm_vcpu_block(), the task will
>>> be set in TASK_INTERRUPTIBLE state, kvm_sched_out will set
>>> vcpu->preempted to true iff current->state == TASK_RUNNING.
>>
>> Ok, I was totally blind to that "if" around vcpu->preempted = true, it's
>> obvious now.
>>
>> I think we need two flags then, for example vcpu->preempted and vcpu->ready:
>>
>> - kvm_sched_out sets both of them to true iff current->state == TASK_RUNNING
>>
>> - kvm_vcpu_kick sets vcpu->ready to true
>>
>> - kvm_sched_in clears both of them

... and also kvm_vcpu_on_spin should check vcpu->ready.  vcpu->preempted
remains only for use by vmx_vcpu_pi_put.

Later we could think of removing vcpu->preempted.  For example,
kvm_arch_sched_out and kvm_x86_ops->sched_out could get the code
currently in vmx_vcpu_pi_put (testing curent->state == TASK_RUNNING
instead of vcpu->preempted).  But for now there's no need and I'm not
sure it's an improvement at all.

Paolo

>> This way, vmx_vcpu_pi_load can keep looking at preempted only (it
>> handles voluntary preemption in pi_pre_block/pi_post_block).