> -----邮件原件-----
> 发件人: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 发送时间: 2022年1月13日 5:31
> 收件人: Sean Christopherson <seanjc@xxxxxxxxxx>
> 抄送: Li,Rongqing <lirongqing@xxxxxxxxx>; pbonzini@xxxxxxxxxx;
> vkuznets@xxxxxxxxxx; wanpengli@xxxxxxxxxxx; jmattson@xxxxxxxxxx;
> tglx@xxxxxxxxxxxxx; bp@xxxxxxxxx; x86@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> joro@xxxxxxxxxx
> 主题: Re: [PATCH] KVM: X86: set vcpu preempted only if it is preempted
>
> On Wed, Jan 12, 2022 at 05:30:47PM +0000, Sean Christopherson wrote:
> > On Wed, Jan 12, 2022, Peter Zijlstra wrote:
> > > On Wed, Jan 12, 2022 at 08:02:01PM +0800, Li RongQing wrote:
> > > > vcpu can schedule out when run halt instruction, and set itself to
> > > > INTERRUPTIBLE and switch to idle thread, vcpu should not be set
> > > > preempted for this condition
> > >
> > > Uhhmm, why not? Who says the vcpu will run the moment it becomes
> > > runnable again? Another task could be woken up meanwhile occupying
> > > the real cpu.
> >
> > Hrm, but when emulating HLT, e.g. for an idling vCPU, KVM will
> > voluntarily schedule out the vCPU and mark it as preempted from the
> > guest's perspective. The vast majority, probably all, usage of
> > steal_time.preempted expects it to truly mean "preempted" as opposed to
> "not running".
>
> No, the original use-case was locking and that really cares about running.
>
> If the vCPU isn't running, we must not busy-wait for it etc..
>
> Similar to the scheduler use of it, if the vCPU isn't running, we should not
> consider it so. Getting the vCPU task scheduled back on the CPU can take a 'long'
> time.
>
> If you have pinned vCPU threads and no overcommit, we have other knobs to
> indicate this I think.
Is it possible if guest has KVM_HINTS_REALTIME feature, but its HLT instruction is emulated by KVM?
If it is possible, this condition has been performance degradation, since vcpu_is_preempted is not __kvm_vcpu_is_preempted, will return false.
Similar, guest has nopvspin, but HLT instruction is emulated;
Should we adjust the setting of pv_ops.lock.vcpu_is_preempted as below
And I see the performance boost when guest has nopvspin, but HLT instruction is emulated with below change
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 59abbda..b061d17 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -1048,6 +1048,11 @@ void __init kvm_spinlock_init(void)
return;
}
+ if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
+ pv_ops.lock.vcpu_is_preempted =
+ PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
+ }
+
/*
* Disable PV spinlocks and use native qspinlock when dedicated pCPUs
* are available.
@@ -1076,10 +1081,6 @@ void __init kvm_spinlock_init(void)
pv_ops.lock.wait = kvm_wait;
pv_ops.lock.kick = kvm_kick_cpu;
- if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
- pv_ops.lock.vcpu_is_preempted =
- PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
- }
/*
* When PV spinlock is enabled which is preferred over
* virt_spin_lock(), virt_spin_lock_key's value is meaningless.
-Li
