[PATCH v2 3/3] KVM: vCPU kick tax cut for running vCPU

Wanpeng Li
2021-10-09T09:09:51Z

Sometimes a vCPU kick is following a pending request, even if @vcpu is
the running vCPU. It suffers from both rcuwait_wake_up() which has
rcu/memory barrier operations and cmpxchg(). Let's check vcpu->wait
before rcu_wait_wake_up() and whether @vcpu is the running vCPU before
cmpxchg() to tax cut this overhead.

We evaluate the kvm-unit-test/vmexit.flat on an Intel ICX box, most of the
scores can improve ~600 cpu cycles especially when APICv is disabled.

tscdeadline_immed
tscdeadline
self_ipi_sti_nop
..............
x2apic_self_ipi_tpr_sti_hlt

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
v1 -> v2:
* move checking running vCPU logic to kvm_vcpu_kick
* check rcuwait_active(&vcpu->wait) etc

virt/kvm/kvm_main.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7851f3a1b5f7..18209d7b3711 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3314,8 +3314,15 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
{
int me, cpu;

- if (kvm_vcpu_wake_up(vcpu))
- return;
+ me = get_cpu();
+
+ if (rcuwait_active(&vcpu->wait) && kvm_vcpu_wake_up(vcpu))
+ goto out;
+
+ if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
+ WARN_ON_ONCE(vcpu->mode == IN_GUEST_MODE);
+ goto out;
+ }

/*
* Note, the vCPU could get migrated to a different pCPU at any point
@@ -3324,12 +3331,12 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
* IPI is to force the vCPU to leave IN_GUEST_MODE, and migrating the
* vCPU also requires it to leave IN_GUEST_MODE.
*/
- me = get_cpu();
if (kvm_arch_vcpu_should_kick(vcpu)) {
cpu = READ_ONCE(vcpu->cpu);
if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
smp_send_reschedule(cpu);
}
+out:
put_cpu();
}
EXPORT_SYMBOL_GPL(kvm_vcpu_kick);
--
2.25.1