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    SubjectRe: [PATCH] kvm: x86: make lapic hrtimer pinned
    On Tue, 5 Apr 2016 14:18:01 +0800
    Yang Zhang <yang.zhang.wz@gmail.com> wrote:

    > On 2016/4/5 5:00, Rik van Riel wrote:
    > > On Mon, 2016-04-04 at 16:46 -0400, Luiz Capitulino wrote:
    > >> When a vCPU runs on a nohz_full core, the hrtimer used by
    > >> the lapic emulation code can be migrated to another core.
    > >> When this happens, it's possible to observe milisecond
    > >> latency when delivering timer IRQs to KVM guests.
    > >>
    > >> The huge latency is mainly due to the fact that
    > >> apic_timer_fn() expects to run during a kvm exit. It
    > >> sets KVM_REQ_PENDING_TIMER and let it be handled on kvm
    > >> entry. However, if the timer fires on a different core,
    > >> we have to wait until the next kvm exit for the guest
    > >> to see KVM_REQ_PENDING_TIMER set.
    > >>
    > >> This problem became visible after commit 9642d18ee. This
    > >> commit changed the timer migration code to always attempt
    > >> to migrate timers away from nohz_full cores. While it's
    > >> discussable if this is correct/desirable (I don't think
    > >> it is), it's clear that the lapic emulation code has
    > >> a requirement on firing the hrtimer in the same core
    > >> where it was started. This is achieved by making the
    > >> hrtimer pinned.
    > >
    > > Given that delivering a timer to a guest seems to
    > > involve trapping from the guest to the host, anyway,
    > > I don't see a downside to your patch.
    > >
    > > If that is ever changed (eg. allowing delivery of
    > > a timer interrupt to a VCPU without trapping to the
    > > host), we may want to revisit this.
    >
    >
    > Posted interrupt helps in this case. Currently, KVM doesn't use PI for
    > lapic timer is due to same affinity for lapic timer and VCPU. Now, we
    > can change to use PI for lapic timer. The only concern is what's
    > frequency of timer migration in upstream Linux? If it is frequently,
    > will it bring additional cost?

    I can't answer this questions.

    > BTW, in what case the migration of timers during VCPU scheduling will fail?

    For hrtimers (which is the lapic emulation case), it only succeeds if
    the destination core has a hrtimer expiring before the hrtimer being
    migrated.

    Also, if the hrtimer callback function is already running (that is,
    the timer fired already) it's not migrated either. But I _guess_ this
    case doesn't affect KVM (and there's no much do about it anyways).

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