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>

diff --git a/Documentation/virtual/kvm/halt-polling.txt b/Documentation/virtual/kvm/halt-polling.txt
index 4a841831876978cbc5396f8fc88232815cfd786c..4f791b128dd27a0ed9bc4ad79eddc8794bcab2bd 100644 (file)
--- a/Documentation/virtual/kvm/halt-polling.txt
+++ b/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:

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index e316a2ddb70b9c2d69d5c09638ae2d32be2f35f8..29ffc99bd79bf103af62ea7ce4537e39f4197c21 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/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;
 }

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 85c0c00d51594471aac0a3bc4a9c6455d2cd38be..9d55c63db09b5dcb9ac997d802cb00ff356d4353 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/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 {

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9c8a8bf6e68611a9636e5a0a1be39e6cf60f0ef9..ae818d27a1a4c7a4fce97698c59f3c245c52bf75 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/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;