[PATCH v2 2/3] KVM: dynamic halt_poll_ns adjustment

[Wanpeng Li]

There is a downside of halt_poll_ns since poll is still happen for idle 
VCPU which can waste cpu usage. This patch adds the ability to adjust 
halt_poll_ns dynamically. 

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and 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. The shrink/grow 
matrix is suggested by David: 

if (poll successfully for interrupt): stay the same
  else if (length of kvm_vcpu_block is longer than halt_poll_ns_max): shrink
  else if (length of kvm_vcpu_block is less than halt_poll_ns_max): grow

  halt_poll_ns_shrink/ |
  halt_poll_ns_grow    | grow halt_poll_ns    | shrink halt_poll_ns 
  ---------------------+----------------------+-------------------
  < 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

Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 64 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 93db833..2a4962b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -66,9 +66,26 @@
 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 2000000
+
+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 unsigned 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 . */
+static unsigned int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
+module_param(halt_poll_ns_shrink, int, S_IRUGO);
+
+/* halt polling only reduces halt latency by 10-15 us, 2ms is enough */
+static 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 +1924,48 @@ 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_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);
+}
+
 static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
 {
 	if (kvm_arch_vcpu_runnable(vcpu)) {
@@ -1954,6 +2013,10 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 			break;
 
 		waited = true;
+		if (vcpu->halt_poll_ns > halt_poll_ns_max)
+			shrink_halt_poll_ns(vcpu);
+		else
+			grow_halt_poll_ns(vcpu);
 		schedule();
 	}
 
-- 
1.9.1

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