Re: [PATCH v2 3/5] KVM: X86: Boost vCPU which is in critical section

[Sean Christopherson <seanjc@google.com>]

+tglx and PeterZ

On Fri, Apr 01, 2022, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> The missing semantic gap that occurs when a guest OS is preempted 
> when executing its own critical section, this leads to degradation 
> of application scalability. We try to bridge this semantic gap in 
> some ways, by passing guest preempt_count to the host and checking 
> guest irq disable state, the hypervisor now knows whether guest 
> OSes are running in the critical section, the hypervisor yield-on-spin 
> heuristics can be more smart this time to boost the vCPU candidate 
> who is in the critical section to mitigate this preemption problem, 
> in addition, it is more likely to be a potential lock holder.
> 
> Testing on 96 HT 2 socket Xeon CLX server, with 96 vCPUs VM 100GB RAM,
> one VM running benchmark, the other(none-2) VMs running cpu-bound 
> workloads, There is no performance regression for other benchmarks 
> like Unixbench etc.

...

> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/x86.c       | 22 ++++++++++++++++++++++
>  include/linux/kvm_host.h |  1 +
>  virt/kvm/kvm_main.c      |  7 +++++++
>  3 files changed, 30 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9aa05f79b743..b613cd2b822a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10377,6 +10377,28 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>  	return r;
>  }
>  
> +static bool kvm_vcpu_is_preemptible(struct kvm_vcpu *vcpu)
> +{
> +	int count;
> +
> +	if (!vcpu->arch.pv_pc.preempt_count_enabled)
> +		return false;
> +
> +	if (!kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.pv_pc.preempt_count_cache,
> +	    &count, sizeof(int)))
> +		return !(count & ~PREEMPT_NEED_RESCHED);

As I pointed out in v1[*], this makes PREEMPT_NEED_RESCHED and really the entire
__preempt_count to some extent, KVM guest/host ABI.  That needs acks from sched
folks, and if they're ok with it, needs to be formalized somewhere in kvm_para.h,
not buried in the KVM host code.

[*] https://lore.kernel.org/all/YkOfJeXm8MiMOEyh@google.com

> +
> +	return false;
> +}
> +