[PATCH v4 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 patchset add the ability to adjust 
halt_poll_ns dynamically, grows halt_poll_ns if an interrupt arrives and 
shrinks halt_poll_ns when idle VCPU is detected.

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink. 

Test w/ high cpu overcommit ratio, pin vCPUs, and the halt_poll_ns of 
halt-poll is the default 500000ns, the max halt_poll_ns of dynamic 
halt-poll is 2ms. Then watch the %C0 in the dump of Powertop tool.
The test method is almost from David.

+-----------------+----------------+-------------------+
|                 |                |                   |
|  w/o halt-poll  |  w/ halt-poll  | dynamic halt-poll |
+-----------------+----------------+-------------------+
|                 |                |                   |
|    ~0.9%        |    ~1.8%       |     ~1.2%         |
+-----------------+----------------+-------------------+
                                             
The always halt-poll will increase ~0.9% cpu usage for idle vCPUs and the 
dynamic halt-poll drop it to ~0.3% which means that reduce the 67% overhead 
introduced by always halt-poll.

Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c06e57c..d63790d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -66,9 +66,18 @@
 MODULE_AUTHOR("Qumranet");
 MODULE_LICENSE("GPL");
 
-static unsigned int halt_poll_ns;
+/* halt polling only reduces halt latency by 5-7 us, 2ms is enough */
+static unsigned int halt_poll_ns = 2000000;
 module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
 
+/* Default doubles per-vcpu halt_poll_ns. */
+static unsigned int halt_poll_ns_grow = 2;
+module_param(halt_poll_ns_grow, int, S_IRUGO);
+
+/* Default resets per-vcpu halt_poll_ns . */
+static unsigned int halt_poll_ns_shrink;
+module_param(halt_poll_ns_shrink, int, S_IRUGO);
+
 /*
  * Ordering of locks:
  *
@@ -1907,6 +1916,31 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
 
+static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+	int val = vcpu->halt_poll_ns;
+
+	/* 500us step */
+	if (val == 0 && halt_poll_ns_grow)
+		val = 500000;
+	else
+		val *= halt_poll_ns_grow;
+
+	vcpu->halt_poll_ns = val;
+}
+
+static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+	int val = vcpu->halt_poll_ns;
+
+	if (halt_poll_ns_shrink == 0)
+		val = 0;
+	else
+		val /= halt_poll_ns_shrink;
+
+	vcpu->halt_poll_ns = val;
+}
+
 static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
 {
 	if (kvm_arch_vcpu_runnable(vcpu)) {
@@ -1961,6 +1995,11 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 	cur = ktime_get();
 
 out:
+	if (waited && vcpu->halt_poll_ns > 0)
+		shrink_halt_poll_ns(vcpu);
+	else if (vcpu->halt_poll_ns < halt_poll_ns)
+		grow_halt_poll_ns(vcpu);
+
 	trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_block);
-- 
1.9.1

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