KVM: Boost vCPUs that are delivering interrupts

Inspired by commit 9cac38d (KVM/s390: Set preempted flag during
vcpu wakeup and interrupt delivery), we want to also boost not just
lock holders but also vCPUs that are delivering interrupts. Most
smp_call_function_many calls are synchronous, so the IPI target vCPUs
are also good yield candidates.  This patch introduces vcpu->ready to
boost vCPUs during wakeup and interrupt delivery time; unlike s390 we do
not reuse vcpu->preempted so that voluntarily preempted vCPUs are taken
into account by kvm_vcpu_on_spin, but vmx_vcpu_pi_put is not affected
(VT-d PI handles voluntary preemption separately, in pi_pre_block).

Testing on 80 HT 2 socket Xeon Skylake server, with 80 vCPUs VM 80GB RAM:
ebizzy -M

            vanilla     boosting    improved
1VM          21443       23520         9%
2VM           2800        8000       180%
3VM           1800        3100        72%

Testing on my Haswell desktop 8 HT, with 8 vCPUs VM 8GB RAM, two VMs,
one running ebizzy -M, the other running 'stress --cpu 2':

w/ boosting + w/o pv sched yield(vanilla)

            vanilla     boosting   improved
              1570         4000      155%

w/ boosting + w/ pv sched yield(vanilla)

            vanilla     boosting   improved
              1844         5157      179%

w/o boosting, perf top in VM:

 72.33%  [kernel]       [k] smp_call_function_many
  4.22%  [kernel]       [k] call_function_i
  3.71%  [kernel]       [k] async_page_fault

w/ boosting, perf top in VM:

 38.43%  [kernel]       [k] smp_call_function_many
  6.31%  [kernel]       [k] async_page_fault
  6.13%  libc-2.23.so   [.] __memcpy_avx_unaligned
  4.88%  [kernel]       [k] call_function_interrupt

Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Marc Zyngier <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

arch/s390/kvm/interrupt.c

@@ -1240,7 +1240,7 @@ void kvm_s390_vcpu_wakeup(struct kvm_vcpu *vcpu)
 		 * The vcpu gave up the cpu voluntarily, mark it as a good
 		 * yield-candidate.
 		 */
-		vcpu->preempted = true;
+		vcpu->ready = true;
 		swake_up_one(&vcpu->wq);
 		vcpu->stat.halt_wakeup++;
 	}

include/linux/kvm_host.h

@@ -318,6 +318,7 @@ struct kvm_vcpu {
 	} spin_loop;
 #endif
 	bool preempted;
+	bool ready;
 	struct kvm_vcpu_arch arch;
 	struct dentry *debugfs_dentry;
 };

virt/kvm/kvm_main.c

@@ -314,6 +314,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
 	kvm_vcpu_set_in_spin_loop(vcpu, false);
 	kvm_vcpu_set_dy_eligible(vcpu, false);
 	vcpu->preempted = false;
+	vcpu->ready = false;
 
 	r = kvm_arch_vcpu_init(vcpu);
 	if (r < 0)
@@ -2387,6 +2388,7 @@ bool kvm_vcpu_wake_up(struct kvm_vcpu *vcpu)
 	wqp = kvm_arch_vcpu_wq(vcpu);
 	if (swq_has_sleeper(wqp)) {
 		swake_up_one(wqp);
+		WRITE_ONCE(vcpu->ready, true);
 		++vcpu->stat.halt_wakeup;
 		return true;
 	}
@@ -2500,7 +2502,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 				continue;
 			} else if (pass && i > last_boosted_vcpu)
 				break;
-			if (!READ_ONCE(vcpu->preempted))
+			if (!READ_ONCE(vcpu->ready))
 				continue;
 			if (vcpu == me)
 				continue;
@@ -4203,8 +4205,8 @@ static void kvm_sched_in(struct preempt_notifier *pn, int cpu)
 {
 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
 
-	if (vcpu->preempted)
-		vcpu->preempted = false;
+	vcpu->preempted = false;
+	WRITE_ONCE(vcpu->ready, false);
 
 	kvm_arch_sched_in(vcpu, cpu);
 
@@ -4216,8 +4218,10 @@ static void kvm_sched_out(struct preempt_notifier *pn,
 {
 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
 
-	if (current->state == TASK_RUNNING)
+	if (current->state == TASK_RUNNING) {
 		vcpu->preempted = true;
+		WRITE_ONCE(vcpu->ready, true);
+	}
 	kvm_arch_vcpu_put(vcpu);
 }