[PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler

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

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

When total number of VCPUs of system is less than or equal to physical CPUs,
PLE exits become costly since each VCPU can have dedicated PCPU, and
trying to find a target VCPU to yield_to just burns time in PLE handler.

This patch reduces overhead, by simply doing a return in such scenarios by
checking the length of current cpu runqueue.

Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
---
 include/linux/sched.h |    1 +
 kernel/sched/core.c   |    6 ++++++
 virt/kvm/kvm_main.c   |    3 +++
 3 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b8c8664..3645458 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -138,6 +138,7 @@ extern int nr_threads;
 DECLARE_PER_CPU(unsigned long, process_counts);
 extern int nr_processes(void);
 extern unsigned long nr_running(void);
+extern unsigned long rq_nr_running(void);
 extern unsigned long nr_uninterruptible(void);
 extern unsigned long nr_iowait(void);
 extern unsigned long nr_iowait_cpu(int cpu);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fbf1fd0..2170b81 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4820,6 +4820,12 @@ void __sched yield(void)
 }
 EXPORT_SYMBOL(yield);
 
+unsigned long rq_nr_running(void)
+{
+	return this_rq()->nr_running;
+}
+EXPORT_SYMBOL(rq_nr_running);
+
 /**
  * yield_to - yield the current processor to another thread in
  * your thread group, or accelerate that thread toward the
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 28f00bc..8323685 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1629,6 +1629,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 	int pass;
 	int i;
 
+	if (unlikely(rq_nr_running() == 1))
+		return;
+
 	kvm_vcpu_set_in_spin_loop(me, true);
 	/*
 	 * We boost the priority of a VCPU that is runnable but not