[PATCH] KVM: halt_polling: allow tuning of grow/shrink

[PATCH] KVM: halt_polling: allow tuning of grow/shrink

[Christian Borntraeger]

Right now halt_poll_ns can be change during runtime. The
grow and shrink factors can only be set during module load.
Let's make this consistent and allow changes during runtime.
To avoid dirty tricky like setting shrink to 0 after the
check for 0, use READ_ONCE to get a consistent number for
all cases.

Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 virt/kvm/kvm_main.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 726d7c8..eafea6f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -72,11 +72,11 @@ 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);
+module_param(halt_poll_ns_grow, int, S_IRUGO | S_IWUSR);
 
 /* Default resets per-vcpu halt_poll_ns . */
 static unsigned int halt_poll_ns_shrink;
-module_param(halt_poll_ns_shrink, int, S_IRUGO);
+module_param(halt_poll_ns_shrink, int, S_IRUGO | S_IWUSR);
 
 /*
  * Ordering of locks:
@@ -1952,14 +1952,15 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
 
 static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-	int old, val;
+	int old, val, grow;
 
-	old = val = vcpu->halt_poll_ns;
+	old = val = READ_ONCE(vcpu->halt_poll_ns);
+	grow = READ_ONCE(halt_poll_ns_grow);
 	/* 10us base */
-	if (val == 0 && halt_poll_ns_grow)
+	if (val == 0 && grow)
 		val = 10000;
 	else
-		val *= halt_poll_ns_grow;
+		val *= grow;
 
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
@@ -1967,13 +1968,14 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 
 static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-	int old, val;
+	int old, val, shrink;
 
-	old = val = vcpu->halt_poll_ns;
-	if (halt_poll_ns_shrink == 0)
+	old = val = READ_ONCE(vcpu->halt_poll_ns);
+	shrink = READ_ONCE(halt_poll_ns_shrink);
+	if (shrink == 0)
 		val = 0;
 	else
-		val /= halt_poll_ns_shrink;
+		val /= shrink;
 
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_shrink(vcpu->vcpu_id, val, old);
-- 
2.3.0

Re: [PATCH] KVM: halt_polling: allow tuning of grow/shrink

[Christian Borntraeger]

On 02/09/2016 01:00 PM, Christian Borntraeger wrote:
> Right now halt_poll_ns can be change during runtime. The
> grow and shrink factors can only be set during module load.
> Let's make this consistent and allow changes during runtime.
> To avoid dirty tricky like setting shrink to 0 after the
> check for 0, use READ_ONCE to get a consistent number for
> all cases.
> 
> Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  virt/kvm/kvm_main.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 726d7c8..eafea6f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -72,11 +72,11 @@ 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);
> +module_param(halt_poll_ns_grow, int, S_IRUGO | S_IWUSR);

Please let me know if I should respin with a global fixup
regarding signedness:

Right now we have a mix of
uint:
>  static unsigned int halt_poll_ns_grow = 2;
int:
> +module_param(halt_poll_ns_grow, int, S_IRUGO | S_IWUSR);
int:
> -	int old, val;

I think having everything as unsigned int would be
the right thing.

Christian