[PATCH 2/3] KVM: dynamise halt_poll_ns adjustment

[Wanpeng Li]

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink. halt_poll_ns_grow affects 
halt_poll_ns when an interrupt arrives and halt_poll_ns_shrink
does it when idle VCPU is detected. 

  halt_poll_ns_shrink/ |
  halt_poll_ns_grow    | interrupt arrives    | idle VCPU is detected
  ---------------------+----------------------+-------------------
  < 1                  |  = halt_poll_ns      |  = 0 
  < halt_poll_ns       | *= halt_poll_ns_grow | /= halt_poll_ns_shrink
  otherwise            | += halt_poll_ns_grow | -= halt_poll_ns_shrink

A third new parameter, halt_poll_ns_max, controls the maximal halt_poll_ns;
it is internally rounded down to a closest multiple of halt_poll_ns_grow.

Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 80 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a122b52..bcfbd35 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -66,9 +66,28 @@
 MODULE_AUTHOR("Qumranet");
 MODULE_LICENSE("GPL");
 
-static unsigned int halt_poll_ns;
+#define KVM_HALT_POLL_NS  500000
+#define KVM_HALT_POLL_NS_GROW   2
+#define KVM_HALT_POLL_NS_SHRINK 0
+#define KVM_HALT_POLL_NS_MAX \
+		INT_MAX / KVM_HALT_POLL_NS_GROW
+
+static unsigned int halt_poll_ns = KVM_HALT_POLL_NS;
 module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
 
+/* Default doubles per-vcpu halt_poll_ns. */
+static int halt_poll_ns_grow = KVM_HALT_POLL_NS_GROW;
+module_param(halt_poll_ns_grow, int, S_IRUGO);
+
+/* Default resets per-vcpu halt_poll_ns . */
+int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
+module_param(halt_poll_ns_shrink, int, S_IRUGO);
+
+/* Default is to compute the maximum so we can never overflow. */
+unsigned int halt_poll_ns_actual_max = KVM_HALT_POLL_NS_MAX;
+unsigned int halt_poll_ns_max = KVM_HALT_POLL_NS_MAX;
+module_param(halt_poll_ns_max, int, S_IRUGO);
+
 /*
  * Ordering of locks:
  *
@@ -1907,6 +1926,62 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
 
+static unsigned int __grow_halt_poll_ns(unsigned int val)
+{
+	if (halt_poll_ns_grow < 1)
+		return halt_poll_ns;
+
+	val = min(val, halt_poll_ns_actual_max);
+
+	if (val == 0)
+		return halt_poll_ns;
+
+	if (halt_poll_ns_grow < halt_poll_ns)
+		val *= halt_poll_ns_grow;
+	else
+		val += halt_poll_ns_grow;
+
+	return val;
+}
+
+static unsigned int __shrink_halt_poll_ns(int val, int modifier, int minimum)
+{
+	if (modifier < 1)
+		return 0;
+
+	if (modifier < halt_poll_ns)
+		val /= modifier;
+	else
+		val -= modifier;
+
+	return val;
+}
+
+static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+	vcpu->halt_poll_ns = __grow_halt_poll_ns(vcpu->halt_poll_ns);
+}
+
+static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+	vcpu->halt_poll_ns = __shrink_halt_poll_ns(vcpu->halt_poll_ns,
+			halt_poll_ns_shrink, halt_poll_ns);
+}
+
+/*
+ * halt_poll_ns_actual_max is computed to be one grow_halt_poll_ns() below
+ * halt_poll_ns_max. (See __grow_halt_poll_ns for the reason.)
+ * This prevents overflows, because ple_halt_poll_ns is int.
+ * halt_poll_ns_max effectively rounded down to a multiple of halt_poll_ns_grow in
+ * this process.
+ */
+static void update_halt_poll_ns_actual_max(void)
+{
+	halt_poll_ns_actual_max =
+		__shrink_halt_poll_ns(max(halt_poll_ns_max, halt_poll_ns),
+			halt_poll_ns_grow, INT_MIN);
+}
+
 static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
 {
 	if (kvm_arch_vcpu_runnable(vcpu)) {
@@ -1941,6 +2016,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 			 */
 			if (kvm_vcpu_check_block(vcpu) < 0) {
 				++vcpu->stat.halt_successful_poll;
+				grow_halt_poll_ns(vcpu);
 				goto out;
 			}
 			cur = ktime_get();
@@ -1954,6 +2030,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 			break;
 
 		waited = true;
+		if (halt_poll_ns)
+			shrink_halt_poll_ns(vcpu);
 		schedule();
 	}
 
@@ -2950,6 +3028,7 @@ static void hardware_enable_nolock(void *junk)
 	cpumask_set_cpu(cpu, cpus_hardware_enabled);
 
 	r = kvm_arch_hardware_enable();
+	update_halt_poll_ns_actual_max();
 
 	if (r) {
 		cpumask_clear_cpu(cpu, cpus_hardware_enabled);
-- 
1.9.1

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