Re: [PATCH v3 0/4] KVM: LAPIC: Implement Exitless Timer

[Maxim Levitsky]

On Thu, 2019-06-13 at 16:25 +0800, Wanpeng Li wrote:
> On Thu, 13 Jun 2019 at 15:59, Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
> > 
> > On Tue, 2019-06-11 at 20:17 +0800, Wanpeng Li wrote:
> > > Dedicated instances are currently disturbed by unnecessary jitter due
> > > to the emulated lapic timers fire on the same pCPUs which vCPUs resident.
> > > There is no hardware virtual timer on Intel for guest like ARM. Both
> > > programming timer in guest and the emulated timer fires incur vmexits.
> > > This patchset tries to avoid vmexit which is incurred by the emulated
> > > timer fires in dedicated instance scenario.
> > > 
> > > When nohz_full is enabled in dedicated instances scenario, the unpinned
> > > timer will be moved to the nearest busy housekeepers after commit 444969223c8
> > > ("sched/nohz: Fix affine unpinned timers mess"). However, KVM always makes
> > > lapic timer pinned to the pCPU which vCPU residents, the reason is explained
> > > by commit 61abdbe0 (kvm: x86: make lapic hrtimer pinned). Actually, these
> > > emulated timers can be offload to the housekeeping cpus since APICv
> > > is really common in recent years. The guest timer interrupt is injected by
> > > posted-interrupt which is delivered by housekeeping cpu once the emulated
> > > timer fires.
> > > 
> > > 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.
> > > ~3% redis performance benefit can be observed on Skylake server.
> > 
> > I don't yet know the kvm well enough to review this patch series, but overall I really like the idea.
> 
> Thank you. :)
> 
> > I researched this area some time ago, to see what can be done to reduce the number of vmexits,
> > to an absolute minimum.
> > 
> > I have one small question, just out of curiosity.
> > 
> > Why do you require mwait in the guest to be enabled?
> > 
> > If I understand it correctly, you say
> > that when mwait in the guest is disabled, then vmx preemption timer will be used,
> > and thus it will handle the apic timer?
> 
> Actually we don't have this restriction in v3, the patchset
> description need to update. The lapic timer which guest use can be
> emulated by software(a hrtimer on host) or VT-x hardware (VMX
> preemption timer). VMX preemption timer triggers vmexit when the timer
> fires on the same pCPU which vCPU is running on, so the injection
> vmexit can't be avoided. The hrtimer on host is used to emulate the
> lapic timer when VMX preemption timer is disabled. After commit
> 9642d18eee2cd(nohz: Affine unpinned timers to housekeepers), unpinned
> timers will be moved to nearest busy housekeepers, which means that we
> can offload the hrtimer to the housekeeping cpus instead of running on
> the pCPU which vCPU residents, the timer fires on the housekeeping
> cpus and be injected by posted-interrupt to the vCPU w/o incur
> vmexits. In patchset v3, the preemption timer will be disarmed if
> lapic timer is injected by posted-interrupt. VMX preemption timer stop
> working in C-states deeper than C2, that's why I utilize mwait expose
> before. (commit 386c6ddbda1 X86/VMX: Disable VMX preemption timer if
> MWAIT is not intercepted)

That explains it very well. Thank you!

Best regards,
	Maxim Levitsky