There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink. halt_poll_ns_grow affects
halt_poll_ns when an interrupt arrives and halt_poll_ns_shrink
does it when idle VCPU is detected.
halt_poll_ns_shrink/ |
halt_poll_ns_grow | interrupt arrives | idle VCPU is detected
---------------------+----------------------+-------------------
< 1 | = halt_poll_ns | = 0
< halt_poll_ns | *= halt_poll_ns_grow | /= halt_poll_ns_shrink
otherwise | += halt_poll_ns_grow | -= halt_poll_ns_shrink
A third new parameter, halt_poll_ns_max, controls the maximal halt_poll_ns;
it is internally rounded down to a closest multiple of halt_poll_ns_grow.
Signed-off-by: Wanpeng Li <[email protected]>
---
virt/kvm/kvm_main.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 80 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a122b52..bcfbd35 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -66,9 +66,28 @@
MODULE_AUTHOR("Qumranet");
MODULE_LICENSE("GPL");
-static unsigned int halt_poll_ns;
+#define KVM_HALT_POLL_NS 500000
+#define KVM_HALT_POLL_NS_GROW 2
+#define KVM_HALT_POLL_NS_SHRINK 0
+#define KVM_HALT_POLL_NS_MAX \
+ INT_MAX / KVM_HALT_POLL_NS_GROW
+
+static unsigned int halt_poll_ns = KVM_HALT_POLL_NS;
module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
+/* Default doubles per-vcpu halt_poll_ns. */
+static int halt_poll_ns_grow = KVM_HALT_POLL_NS_GROW;
+module_param(halt_poll_ns_grow, int, S_IRUGO);
+
+/* Default resets per-vcpu halt_poll_ns . */
+int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
+module_param(halt_poll_ns_shrink, int, S_IRUGO);
+
+/* Default is to compute the maximum so we can never overflow. */
+unsigned int halt_poll_ns_actual_max = KVM_HALT_POLL_NS_MAX;
+unsigned int halt_poll_ns_max = KVM_HALT_POLL_NS_MAX;
+module_param(halt_poll_ns_max, int, S_IRUGO);
+
/*
* Ordering of locks:
*
@@ -1907,6 +1926,62 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
}
EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
+static unsigned int __grow_halt_poll_ns(unsigned int val)
+{
+ if (halt_poll_ns_grow < 1)
+ return halt_poll_ns;
+
+ val = min(val, halt_poll_ns_actual_max);
+
+ if (val == 0)
+ return halt_poll_ns;
+
+ if (halt_poll_ns_grow < halt_poll_ns)
+ val *= halt_poll_ns_grow;
+ else
+ val += halt_poll_ns_grow;
+
+ return val;
+}
+
+static unsigned int __shrink_halt_poll_ns(int val, int modifier, int minimum)
+{
+ if (modifier < 1)
+ return 0;
+
+ if (modifier < halt_poll_ns)
+ val /= modifier;
+ else
+ val -= modifier;
+
+ return val;
+}
+
+static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+ vcpu->halt_poll_ns = __grow_halt_poll_ns(vcpu->halt_poll_ns);
+}
+
+static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+ vcpu->halt_poll_ns = __shrink_halt_poll_ns(vcpu->halt_poll_ns,
+ halt_poll_ns_shrink, halt_poll_ns);
+}
+
+/*
+ * halt_poll_ns_actual_max is computed to be one grow_halt_poll_ns() below
+ * halt_poll_ns_max. (See __grow_halt_poll_ns for the reason.)
+ * This prevents overflows, because ple_halt_poll_ns is int.
+ * halt_poll_ns_max effectively rounded down to a multiple of halt_poll_ns_grow in
+ * this process.
+ */
+static void update_halt_poll_ns_actual_max(void)
+{
+ halt_poll_ns_actual_max =
+ __shrink_halt_poll_ns(max(halt_poll_ns_max, halt_poll_ns),
+ halt_poll_ns_grow, INT_MIN);
+}
+
static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
{
if (kvm_arch_vcpu_runnable(vcpu)) {
@@ -1941,6 +2016,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
*/
if (kvm_vcpu_check_block(vcpu) < 0) {
++vcpu->stat.halt_successful_poll;
+ grow_halt_poll_ns(vcpu);
goto out;
}
cur = ktime_get();
@@ -1954,6 +2030,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
break;
waited = true;
+ if (halt_poll_ns)
+ shrink_halt_poll_ns(vcpu);
schedule();
}
@@ -2950,6 +3028,7 @@ static void hardware_enable_nolock(void *junk)
cpumask_set_cpu(cpu, cpus_hardware_enabled);
r = kvm_arch_hardware_enable();
+ update_halt_poll_ns_actual_max();
if (r) {
cpumask_clear_cpu(cpu, cpus_hardware_enabled);
--
1.9.1
On Mon, Aug 24, 2015 at 5:53 AM, Wanpeng Li <[email protected]> wrote:
> There are two new kernel parameters for changing the halt_poll_ns:
> halt_poll_ns_grow and halt_poll_ns_shrink. halt_poll_ns_grow affects
> halt_poll_ns when an interrupt arrives and halt_poll_ns_shrink
> does it when idle VCPU is detected.
>
> halt_poll_ns_shrink/ |
> halt_poll_ns_grow | interrupt arrives | idle VCPU is detected
> ---------------------+----------------------+-------------------
> < 1 | = halt_poll_ns | = 0
> < halt_poll_ns | *= halt_poll_ns_grow | /= halt_poll_ns_shrink
> otherwise | += halt_poll_ns_grow | -= halt_poll_ns_shrink
>
> A third new parameter, halt_poll_ns_max, controls the maximal halt_poll_ns;
> it is internally rounded down to a closest multiple of halt_poll_ns_grow.
I like the idea of growing and shrinking halt_poll_ns, but I'm not sure
we grow and shrink in the right places here. For example, if vcpu->halt_poll_ns
gets down to 0, I don't see how it can then grow back up.
This might work better:
if (poll successfully for interrupt): stay the same
else if (length of kvm_vcpu_block is longer than halt_poll_ns_max): shrink
else if (length of kvm_vcpu_block is less than halt_poll_ns_max): grow
where halt_poll_ns_max is something reasonable, like 2 millisecond.
You get diminishing returns from halt polling as the length of the
halt gets longer (halt polling only reduces halt latency by 10-15 us).
So there's little benefit to polling longer than a few milliseconds.
>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> virt/kvm/kvm_main.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 80 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a122b52..bcfbd35 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -66,9 +66,28 @@
> MODULE_AUTHOR("Qumranet");
> MODULE_LICENSE("GPL");
>
> -static unsigned int halt_poll_ns;
> +#define KVM_HALT_POLL_NS 500000
> +#define KVM_HALT_POLL_NS_GROW 2
> +#define KVM_HALT_POLL_NS_SHRINK 0
> +#define KVM_HALT_POLL_NS_MAX \
> + INT_MAX / KVM_HALT_POLL_NS_GROW
> +
> +static unsigned int halt_poll_ns = KVM_HALT_POLL_NS;
> module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
>
> +/* Default doubles per-vcpu halt_poll_ns. */
> +static int halt_poll_ns_grow = KVM_HALT_POLL_NS_GROW;
> +module_param(halt_poll_ns_grow, int, S_IRUGO);
> +
> +/* Default resets per-vcpu halt_poll_ns . */
> +int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
> +module_param(halt_poll_ns_shrink, int, S_IRUGO);
> +
> +/* Default is to compute the maximum so we can never overflow. */
> +unsigned int halt_poll_ns_actual_max = KVM_HALT_POLL_NS_MAX;
> +unsigned int halt_poll_ns_max = KVM_HALT_POLL_NS_MAX;
> +module_param(halt_poll_ns_max, int, S_IRUGO);
> +
> /*
> * Ordering of locks:
> *
> @@ -1907,6 +1926,62 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
> }
> EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
>
> +static unsigned int __grow_halt_poll_ns(unsigned int val)
> +{
> + if (halt_poll_ns_grow < 1)
> + return halt_poll_ns;
> +
> + val = min(val, halt_poll_ns_actual_max);
> +
> + if (val == 0)
> + return halt_poll_ns;
> +
> + if (halt_poll_ns_grow < halt_poll_ns)
> + val *= halt_poll_ns_grow;
> + else
> + val += halt_poll_ns_grow;
> +
> + return val;
> +}
> +
> +static unsigned int __shrink_halt_poll_ns(int val, int modifier, int minimum)
> +{
> + if (modifier < 1)
> + return 0;
> +
> + if (modifier < halt_poll_ns)
> + val /= modifier;
> + else
> + val -= modifier;
> +
> + return val;
> +}
> +
> +static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
> +{
> + vcpu->halt_poll_ns = __grow_halt_poll_ns(vcpu->halt_poll_ns);
> +}
> +
> +static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
> +{
> + vcpu->halt_poll_ns = __shrink_halt_poll_ns(vcpu->halt_poll_ns,
> + halt_poll_ns_shrink, halt_poll_ns);
> +}
> +
> +/*
> + * halt_poll_ns_actual_max is computed to be one grow_halt_poll_ns() below
> + * halt_poll_ns_max. (See __grow_halt_poll_ns for the reason.)
> + * This prevents overflows, because ple_halt_poll_ns is int.
> + * halt_poll_ns_max effectively rounded down to a multiple of halt_poll_ns_grow in
> + * this process.
> + */
> +static void update_halt_poll_ns_actual_max(void)
> +{
> + halt_poll_ns_actual_max =
> + __shrink_halt_poll_ns(max(halt_poll_ns_max, halt_poll_ns),
> + halt_poll_ns_grow, INT_MIN);
> +}
> +
> static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> {
> if (kvm_arch_vcpu_runnable(vcpu)) {
> @@ -1941,6 +2016,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> */
> if (kvm_vcpu_check_block(vcpu) < 0) {
> ++vcpu->stat.halt_successful_poll;
> + grow_halt_poll_ns(vcpu);
> goto out;
> }
> cur = ktime_get();
> @@ -1954,6 +2030,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> break;
>
> waited = true;
> + if (halt_poll_ns)
> + shrink_halt_poll_ns(vcpu);
> schedule();
> }
>
> @@ -2950,6 +3028,7 @@ static void hardware_enable_nolock(void *junk)
> cpumask_set_cpu(cpu, cpus_hardware_enabled);
>
> r = kvm_arch_hardware_enable();
> + update_halt_poll_ns_actual_max();
>
> if (r) {
> cpumask_clear_cpu(cpu, cpus_hardware_enabled);
> --
> 1.9.1
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to [email protected]
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
Hi David，
On 8/25/15 1:00 AM, David Matlack wrote:
> On Mon, Aug 24, 2015 at 5:53 AM, Wanpeng Li <[email protected]> wrote:
>> There are two new kernel parameters for changing the halt_poll_ns:
>> halt_poll_ns_grow and halt_poll_ns_shrink. halt_poll_ns_grow affects
>> halt_poll_ns when an interrupt arrives and halt_poll_ns_shrink
>> does it when idle VCPU is detected.
>>
>> halt_poll_ns_shrink/ |
>> halt_poll_ns_grow | interrupt arrives | idle VCPU is detected
>> ---------------------+----------------------+-------------------
>> < 1 | = halt_poll_ns | = 0
>> < halt_poll_ns | *= halt_poll_ns_grow | /= halt_poll_ns_shrink
>> otherwise | += halt_poll_ns_grow | -= halt_poll_ns_shrink
>>
>> A third new parameter, halt_poll_ns_max, controls the maximal halt_poll_ns;
>> it is internally rounded down to a closest multiple of halt_poll_ns_grow.
> I like the idea of growing and shrinking halt_poll_ns, but I'm not sure
> we grow and shrink in the right places here. For example, if vcpu->halt_poll_ns
> gets down to 0, I don't see how it can then grow back up.
This has already done in __grow_halt_poll_ns().
>
> This might work better:
> if (poll successfully for interrupt): stay the same
> else if (length of kvm_vcpu_block is longer than halt_poll_ns_max): shrink
> else if (length of kvm_vcpu_block is less than halt_poll_ns_max): grow
>
> where halt_poll_ns_max is something reasonable, like 2 millisecond.
>
> You get diminishing returns from halt polling as the length of the
> halt gets longer (halt polling only reduces halt latency by 10-15 us).
> So there's little benefit to polling longer than a few milliseconds.
Great idea, David! Thanks for your review and I will implement it in
next version. :-)
Regards,
Wanpeng Li
>
>> Signed-off-by: Wanpeng Li <[email protected]>
>> ---
>> virt/kvm/kvm_main.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 80 insertions(+), 1 deletion(-)
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index a122b52..bcfbd35 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -66,9 +66,28 @@
>> MODULE_AUTHOR("Qumranet");
>> MODULE_LICENSE("GPL");
>>
>> -static unsigned int halt_poll_ns;
>> +#define KVM_HALT_POLL_NS 500000
>> +#define KVM_HALT_POLL_NS_GROW 2
>> +#define KVM_HALT_POLL_NS_SHRINK 0
>> +#define KVM_HALT_POLL_NS_MAX \
>> + INT_MAX / KVM_HALT_POLL_NS_GROW
>> +
>> +static unsigned int halt_poll_ns = KVM_HALT_POLL_NS;
>> module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
>>
>> +/* Default doubles per-vcpu halt_poll_ns. */
>> +static int halt_poll_ns_grow = KVM_HALT_POLL_NS_GROW;
>> +module_param(halt_poll_ns_grow, int, S_IRUGO);
>> +
>> +/* Default resets per-vcpu halt_poll_ns . */
>> +int halt_poll_ns_shrink = KVM_HALT_POLL_NS_SHRINK;
>> +module_param(halt_poll_ns_shrink, int, S_IRUGO);
>> +
>> +/* Default is to compute the maximum so we can never overflow. */
>> +unsigned int halt_poll_ns_actual_max = KVM_HALT_POLL_NS_MAX;
>> +unsigned int halt_poll_ns_max = KVM_HALT_POLL_NS_MAX;
>> +module_param(halt_poll_ns_max, int, S_IRUGO);
>> +
>> /*
>> * Ordering of locks:
>> *
>> @@ -1907,6 +1926,62 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
>> }
>> EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
>>
>> +static unsigned int __grow_halt_poll_ns(unsigned int val)
>> +{
>> + if (halt_poll_ns_grow < 1)
>> + return halt_poll_ns;
>> +
>> + val = min(val, halt_poll_ns_actual_max);
>> +
>> + if (val == 0)
>> + return halt_poll_ns;
>> +
>> + if (halt_poll_ns_grow < halt_poll_ns)
>> + val *= halt_poll_ns_grow;
>> + else
>> + val += halt_poll_ns_grow;
>> +
>> + return val;
>> +}
>> +
>> +static unsigned int __shrink_halt_poll_ns(int val, int modifier, int minimum)
>> +{
>> + if (modifier < 1)
>> + return 0;
>> +
>> + if (modifier < halt_poll_ns)
>> + val /= modifier;
>> + else
>> + val -= modifier;
>> +
>> + return val;
>> +}
>> +
>> +static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
>> +{
>> + vcpu->halt_poll_ns = __grow_halt_poll_ns(vcpu->halt_poll_ns);
>> +}
>> +
>> +static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
>> +{
>> + vcpu->halt_poll_ns = __shrink_halt_poll_ns(vcpu->halt_poll_ns,
>> + halt_poll_ns_shrink, halt_poll_ns);
>> +}
>> +
>> +/*
>> + * halt_poll_ns_actual_max is computed to be one grow_halt_poll_ns() below
>> + * halt_poll_ns_max. (See __grow_halt_poll_ns for the reason.)
>> + * This prevents overflows, because ple_halt_poll_ns is int.
>> + * halt_poll_ns_max effectively rounded down to a multiple of halt_poll_ns_grow in
>> + * this process.
>> + */
>> +static void update_halt_poll_ns_actual_max(void)
>> +{
>> + halt_poll_ns_actual_max =
>> + __shrink_halt_poll_ns(max(halt_poll_ns_max, halt_poll_ns),
>> + halt_poll_ns_grow, INT_MIN);
>> +}
>> +
>> static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>> {
>> if (kvm_arch_vcpu_runnable(vcpu)) {
>> @@ -1941,6 +2016,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> */
>> if (kvm_vcpu_check_block(vcpu) < 0) {
>> ++vcpu->stat.halt_successful_poll;
>> + grow_halt_poll_ns(vcpu);
>> goto out;
>> }
>> cur = ktime_get();
>> @@ -1954,6 +2030,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> break;
>>
>> waited = true;
>> + if (halt_poll_ns)
>> + shrink_halt_poll_ns(vcpu);
>> schedule();
>> }
>>
>> @@ -2950,6 +3028,7 @@ static void hardware_enable_nolock(void *junk)
>> cpumask_set_cpu(cpu, cpus_hardware_enabled);
>>
>> r = kvm_arch_hardware_enable();
>> + update_halt_poll_ns_actual_max();
>>
>> if (r) {
>> cpumask_clear_cpu(cpu, cpus_hardware_enabled);
>> --
>> 1.9.1
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to [email protected]
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at http://www.tux.org/lkml/