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

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Message-ID: <bc210153-fbae-25d4-bf6b-e31ceef36aa5@redhat.com>
Date:   Thu, 18 Jul 2019 11:07:03 +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 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

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

Also, kvm_s390_vcpu_wakeup can be changed to use kvm_vcpu_wake_up, which
is nice.

Paolo