linux-kernel - Re: [PATCH v3 1/4] KVM: LAPIC: Make lapic timer unpinned when timer is injected by pi

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Date:   Wed, 12 Jun 2019 08:45:10 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v3 1/4] KVM: LAPIC: Make lapic timer unpinned when timer
 is injected by pi

On Wed, 12 Jun 2019 at 04:39, Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> On Tue, Jun 11, 2019 at 08:17:06PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Make lapic timer unpinned when timer is injected by posted-interrupt,
> > the emulated timer can be offload to the housekeeping cpus.
> >
> > The host admin should fine tuned, e.g. dedicated instances scenario
> > w/ nohz_full cover the pCPUs which vCPUs resident, several pCPUs
> > surplus for housekeeping, disable mwait/hlt/pause vmexits to occupy
> > the pCPUs, fortunately preemption timer is disabled after mwait is
> > exposed to guest which makes emulated timer offload can be possible.
>
> Li,
>
> Nice!
>
> I think you can drop the HRTIMER_MODE_ABS_PINNED and
> instead have
>
> void kvm_set_pending_timer(struct kvm_vcpu *vcpu)
> {
>         kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
>         kvm_vcpu_kick(vcpu);
> }
>
> As an alternative to commit 61abdbe0bcc2b32745ab4479cc550f4c1f518ee2
> (as a first patch in your series).
>
> This will make the logic simpler (and timer migration, for
> nonhousekeeping case, ensures timer is migrated).

Good point. :)

>
> Also, should make this work for non housekeeping case as well.
> (But that can be done later).

The timer fire may cause other vCPUs vmexits for non housekeeping
case(after migrating timers fail during vCPU is scheduled to run in a
different pCPU). Could you explain more?

Regards,
Wanpeng Li