Re: [PATCH v3 0/4] KVM: LAPIC: Implement Exitless Timer
From: Maxim Levitsky
Date: Thu Jun 13 2019 - 12:38:25 EST
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.
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?