Re: [PATCH 2/3] KVM: dynamise halt_poll_ns adjustment

[David Matlack]

On Mon, Aug 24, 2015 at 5:53 AM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> 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 <wanpeng.li@xxxxxxxxxxx>
> ---
>  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
>
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