KVM: Expose the initial start value in grow_halt_poll_ns() as a module parameter

The hard-coded value 10000 in grow_halt_poll_ns() stands for the initial
start value when raising up vcpu->halt_poll_ns.
It actually sets the first timeout to the first polling session.
This value has significant effect on how tolerant we are to outliers.
On the standard case, higher value is better - we will spend more time
in the polling busyloop, handle events/interrupts faster and result
in better performance.
But on outliers it puts us in a busy loop that does nothing.
Even if the shrink factor is zero, we will still waste time on the first
iteration.
The optimal value changes between different workloads. It depends on
outliers rate and polling sessions length.
As this value has significant effect on the dynamic halt-polling
algorithm, it should be configurable and exposed.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Nir Weiner <nir.weiner@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/halt-polling.txt

@@ -53,7 +53,8 @@ the global max polling interval then the polling interval can be increased in
 the hope that next time during the longer polling interval the wake up source
 will be received while the host is polling and the latency benefits will be
 received. The polling interval is grown in the function grow_halt_poll_ns() and
-is multiplied by the module parameter halt_poll_ns_grow.
+is multiplied by the module parameters halt_poll_ns_grow and
+halt_poll_ns_grow_start.
 
 In the event that the total block time was greater than the global max polling
 interval then the host will never poll for long enough (limited by the global
@@ -80,22 +81,30 @@ shrunk. These variables are defined in include/linux/kvm_host.h and as module
 parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
 powerpc kvm-hv case.
 
-Module Parameter    |	     Description	      |	     Default Value
+Module Parameter	|   Description		    |	     Default Value
 --------------------------------------------------------------------------------
-halt_poll_ns	    | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT
-		    | which defines the ceiling value |
-		    | of the polling interval for     | (per arch value)
-		    | each vcpu. 		      |
+halt_poll_ns		| The global max polling    | KVM_HALT_POLL_NS_DEFAULT
+			| interval which defines    |
+			| the ceiling value of the  |
+			| polling interval for      | (per arch value)
+			| each vcpu.		    |
 --------------------------------------------------------------------------------
-halt_poll_ns_grow   | The value by which the halt     |	2
-		    | polling interval is multiplied  |
-		    | in the grow_halt_poll_ns()      |
-		    | function.			      |
+halt_poll_ns_grow	| The value by which the    | 2
+			| halt polling interval is  |
+			| multiplied in the	    |
+			| grow_halt_poll_ns()	    |
+			| function.		    |
 --------------------------------------------------------------------------------
-halt_poll_ns_shrink | The value by which the halt     |	0
-		    | polling interval is divided in  |
-		    | the shrink_halt_poll_ns()	      |
-		    | function.			      |
+halt_poll_ns_grow_start | The initial value to grow | 10000
+			| to from zero in the	    |
+			| grow_halt_poll_ns()	    |
+			| function.		    |
+--------------------------------------------------------------------------------
+halt_poll_ns_shrink	| The value by which the    | 0
+			| halt polling interval is  |
+			| divided in the	    |
+			| shrink_halt_poll_ns()	    |
+			| function.		    |
 --------------------------------------------------------------------------------
 
 These module parameters can be set from the debugfs files in:

arch/powerpc/kvm/book3s_hv.c

@@ -3634,9 +3634,8 @@ static void grow_halt_poll_ns(struct kvmppc_vcore *vc)
 	if (!halt_poll_ns_grow)
 		return;
 
-	/* 10us base */
 	if (vc->halt_poll_ns == 0)
-		vc->halt_poll_ns = 10000;
+		vc->halt_poll_ns = halt_poll_ns_grow_start;
 	else
 		vc->halt_poll_ns *= halt_poll_ns_grow;
 }

include/linux/kvm_host.h

@@ -1203,6 +1203,7 @@ extern bool kvm_rebooting;
 
 extern unsigned int halt_poll_ns;
 extern unsigned int halt_poll_ns_grow;
+extern unsigned int halt_poll_ns_grow_start;
 extern unsigned int halt_poll_ns_shrink;
 
 struct kvm_device {

virt/kvm/kvm_main.c

@@ -81,6 +81,11 @@ unsigned int halt_poll_ns_grow = 2;
 module_param(halt_poll_ns_grow, uint, 0644);
 EXPORT_SYMBOL_GPL(halt_poll_ns_grow);
 
+/* The start value to grow halt_poll_ns from */
+unsigned int halt_poll_ns_grow_start = 10000; /* 10us */
+module_param(halt_poll_ns_grow_start, uint, 0644);
+EXPORT_SYMBOL_GPL(halt_poll_ns_grow_start);
+
 /* Default resets per-vcpu halt_poll_ns . */
 unsigned int halt_poll_ns_shrink;
 module_param(halt_poll_ns_shrink, uint, 0644);
@@ -2191,9 +2196,8 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 	if (!grow)
 		goto out;
 
-	/* 10us base */
 	if (val == 0)
-		val = 10000;
+		val = halt_poll_ns_grow_start;
 	else
 		val *= grow;