linux-kernel - Re: [RFC PATCH 0/2] Mitigating Excessive Pause-Loop Exiting in VM-Agnostic KVM

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Message-ID: <CANRm+Cz9GgQFDbbW_3bRO35FwvMGJ-ZFa0rSvEimxFzrvwmpJw@mail.gmail.com>
Date:   Mon, 26 Apr 2021 11:58:19 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Kenta Ishiguro <kentaishiguro@...ab.ics.keio.ac.jp>
Cc:     David Hildenbrand <david@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        河野健二 <kono@...ab.ics.keio.ac.jp>,
        kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Pierre-Louis Aublin <pl@...ab.ics.keio.ac.jp>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [RFC PATCH 0/2] Mitigating Excessive Pause-Loop Exiting in
 VM-Agnostic KVM

On Mon, 26 Apr 2021 at 11:19, Kenta Ishiguro
<kentaishiguro@...ab.ics.keio.ac.jp> wrote:
>
> Thank you for the reply.
>
> My question is about following scenario:
> 1. running vCPU receives IPI and the vCPU's ipi_received gets true
> 2. the vCPU responds to the IPI
> 3. the vCPU exits
> 4. the vCPU is preempted by KVM
> 5. the vCPU is boosted, but it has already responded to the IPI
> 6. the vCPU enters and the vCPU's ipi_received is cleaned
>
> In this case, I think the check of vcpu->preempted does not limit the candidate vCPUs.

Good point, you are right. However, actually I played with that code a
bit before, I have another version adding the vcpu->preempted checking
when marking IPI receiver, the score is not as good as expected.

    Wanpeng