[PATCH v7 2/3] KVM: dynamic halt-polling

[Wanpeng Li]

There is a downside of always-poll since poll is still happened for idle 
vCPUs which can waste cpu usage. This patchset add the ability to adjust 
halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,  
and to shrink halt_poll_ns when long halt is detected.

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

                        no-poll      always-poll    dynamic-poll
-----------------------------------------------------------------------
Idle (nohz) vCPU %c0     0.15%        0.3%            0.2%  
Idle (250HZ) vCPU %c0    1.1%         4.6%~14%        1.2%
TCP_RR latency           34us         27us            26.7us

"Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
guest was tickless. (250HZ) means the guest was ticking at 250HZ.

The big win is with ticking operating systems. Running the linux guest
with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close 
to no-polling overhead levels by using the dynamic-poll. The savings
should be even higher for higher frequency ticks.

Suggested-by: David Matlack <dmatlack@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 59 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c06e57c..d5e07e9 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, 500us is enough */
+static unsigned int halt_poll_ns = 500000;
 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;
+
+	/* 10us base */
+	if (val == 0 && halt_poll_ns_grow)
+		val = 10000;
+	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)) {
@@ -1928,7 +1962,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 {
 	ktime_t start, cur;
 	DEFINE_WAIT(wait);
-	bool waited = false;
+	bool polled = false, waited = false;
+	u64 poll_ns = 0, wait_ns = 0, block_ns = 0;
 
 	start = cur = ktime_get();
 	if (vcpu->halt_poll_ns) {
@@ -1940,11 +1975,16 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 			 * arrives.
 			 */
 			if (kvm_vcpu_check_block(vcpu) < 0) {
+				polled = true;
 				++vcpu->stat.halt_successful_poll;
-				goto out;
+				break;
 			}
 			cur = ktime_get();
 		} while (single_task_running() && ktime_before(cur, stop));
+
+		poll_ns = ktime_to_ns(cur) - ktime_to_ns(start);
+		if (polled)
+			goto out;
 	}
 
 	for (;;) {
@@ -1959,9 +1999,24 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 
 	finish_wait(&vcpu->wq, &wait);
 	cur = ktime_get();
+	wait_ns = ktime_to_ns(cur) - ktime_to_ns(start);
 
 out:
-	trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
+	block_ns = poll_ns + wait_ns;
+
+	if (halt_poll_ns) {
+		if (block_ns <= vcpu->halt_poll_ns)
+			;
+		/* we had a long block, shrink polling */
+		else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
+			shrink_halt_poll_ns(vcpu);
+		/* we had a short halt and our poll time is too small */
+		else if (vcpu->halt_poll_ns < halt_poll_ns &&
+			block_ns < halt_poll_ns)
+			grow_halt_poll_ns(vcpu);
+	}
+
+	trace_kvm_vcpu_wakeup(block_ns, waited);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_block);
 
-- 
1.9.1

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