Re: [PATCH v5 0/4] KVM: LAPIC: Implement Exitless Timer

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/06/19 11:39, 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
> 9642d18eee2cd (nohz: Affine unpinned timers to housekeepers) and 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 busy housekeeping, disable mwait/hlt/pause vmexits to keep in non-root  
> mode, ~3% redis performance benefit can be observed on Skylake server.

Marcelo,

does this patch work for you or can you still see the oops?

Thanks,

Paolo

> w/o patchset:
> 
>             VM-EXIT  Samples  Samples%  Time%   Min Time  Max Time   Avg time
> 
> EXTERNAL_INTERRUPT    42916    49.43%   39.30%   0.47us   106.09us   0.71us ( +-   1.09% )
> 
> w/ patchset:
> 
>             VM-EXIT  Samples  Samples%  Time%   Min Time  Max Time         Avg time
> 
> EXTERNAL_INTERRUPT    6871     9.29%     2.96%   0.44us    57.88us   0.72us ( +-   4.02% )
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> 
> v4 -> v5:
>  * update patch description in patch 1/4
>  * feed latest apic->lapic_timer.expired_tscdeadline to kvm_wait_lapic_expire()
>  * squash advance timer handling to patch 2/4
> 
> v3 -> v4:
>  * drop the HRTIMER_MODE_ABS_PINNED, add kick after set pending timer
>  * don't posted inject already-expired timer
> 
> v2 -> v3:
>  * disarming the vmx preemption timer when posted_interrupt_inject_timer_enabled()
>  * check kvm_hlt_in_guest instead
> 
> v1 -> v2:
>  * check vcpu_halt_in_guest
>  * move module parameter from kvm-intel to kvm
>  * add housekeeping_enabled
>  * rename apic_timer_expired_pi to kvm_apic_inject_pending_timer_irqs
> 
> 
> Wanpeng Li (4):
>   KVM: LAPIC: Make lapic timer unpinned
>   KVM: LAPIC: Inject timer interrupt via posted interrupt
>   KVM: LAPIC: Ignore timer migration when lapic timer is injected by pi
>   KVM: LAPIC: Don't inject already-expired timer via posted interrupt
> 
>  arch/x86/kvm/lapic.c            | 68 +++++++++++++++++++++++++++--------------
>  arch/x86/kvm/lapic.h            |  3 +-
>  arch/x86/kvm/svm.c              |  2 +-
>  arch/x86/kvm/vmx/vmx.c          |  5 +--
>  arch/x86/kvm/x86.c              | 11 ++++---
>  arch/x86/kvm/x86.h              |  2 ++
>  include/linux/sched/isolation.h |  2 ++
>  kernel/sched/isolation.c        |  6 ++++
>  8 files changed, 67 insertions(+), 32 deletions(-)
>