[PATCH RFC 2/2] kvm PLE handler: Choose better candidate for directed yield

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>

Currently PLE handler can repeatedly do a directed yield to same vcpu
that has recently done PL exit. This can degrade the performance
Try to yield to most eligible guy instead, by alternate yielding.

Precisely, give chance to a VCPU which has:
 (a) Not done PLE exit at all (probably he is preempted lock-holder)
 (b) VCPU skipped in last iteration because it did PL exit, and probably
 has become eligible now (next eligible lock holder)

Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
---
 arch/s390/include/asm/kvm_host.h |    5 +++++
 arch/x86/include/asm/kvm_host.h  |    2 +-
 arch/x86/kvm/x86.c               |   14 ++++++++++++++
 virt/kvm/kvm_main.c              |    3 +++
 4 files changed, 23 insertions(+), 1 deletions(-)

diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index dd17537..884f2c4 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -256,5 +256,10 @@ struct kvm_arch{
 	struct gmap *gmap;
 };
 
+static inline bool kvm_arch_vcpu_check_and_update_eligible(struct kvm_vcpu *v)
+{
+	return true;
+}
+
 extern int sie64a(struct kvm_s390_sie_block *, u64 *);
 #endif
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 857ca68..ce01db3 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -962,7 +962,7 @@ extern bool kvm_find_async_pf_gfn(struct kvm_vcpu *vcpu, gfn_t gfn);
 void kvm_complete_insn_gp(struct kvm_vcpu *vcpu, int err);
 
 int kvm_is_in_guest(void);
-
+bool kvm_arch_vcpu_check_and_update_eligible(struct kvm_vcpu *vcpu);
 void kvm_pmu_init(struct kvm_vcpu *vcpu);
 void kvm_pmu_destroy(struct kvm_vcpu *vcpu);
 void kvm_pmu_reset(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 07dbd14..24ceae8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6623,6 +6623,20 @@ bool kvm_arch_can_inject_async_page_present(struct kvm_vcpu *vcpu)
 			kvm_x86_ops->interrupt_allowed(vcpu);
 }
 
+bool kvm_arch_vcpu_check_and_update_eligible(struct kvm_vcpu *vcpu)
+{
+	bool eligible;
+
+	eligible = !vcpu->arch.plo.pause_loop_exited ||
+			(vcpu->arch.plo.pause_loop_exited &&
+			 vcpu->arch.plo.dy_eligible);
+
+	if (vcpu->arch.plo.pause_loop_exited)
+		vcpu->arch.plo.dy_eligible = !vcpu->arch.plo.dy_eligible;
+
+	return eligible;
+}
+
 EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
 EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_inj_virq);
 EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_page_fault);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7e14068..519321a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1595,6 +1595,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 				continue;
 			if (waitqueue_active(&vcpu->wq))
 				continue;
+			if (!kvm_arch_vcpu_check_and_update_eligible(vcpu)) {
+				continue;
+			}
 			if (kvm_vcpu_yield_to(vcpu)) {
 				kvm->last_boosted_vcpu = i;
 				yielded = 1;