Re: [PATCH v6 2/3] KVM: dynamic halt_poll_ns adjustment

[David Matlack]

On Wed, Sep 2, 2015 at 12:29 AM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> wrote:
> There is a downside of always-poll since poll is still happened for idle
> vCPUs which can waste cpu usage. This patch adds the ability to adjust
> halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,
> and to shrink halt_poll_ns when long halt is detected.
>
> There are two new kernel parameters for changing the halt_poll_ns:
> halt_poll_ns_grow and halt_poll_ns_shrink.
>
>                         no-poll      always-poll    dynamic-poll
> -----------------------------------------------------------------------
> Idle (nohz) vCPU %c0     0.15%        0.3%            0.2%
> Idle (250HZ) vCPU %c0    1.1%         4.6%~14%        1.2%
> TCP_RR latency           34us         27us            26.7us
>
> "Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
> c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
> guest was tickless. (250HZ) means the guest was ticking at 250HZ.
>
> The big win is with ticking operating systems. Running the linux guest
> with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close
> to no-polling overhead levels by using the dynamic-poll. The savings
> should be even higher for higher frequency ticks.
>
> Suggested-by: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  virt/kvm/kvm_main.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 58 insertions(+), 3 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c06e57c..3cff02f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -66,9 +66,18 @@
>  MODULE_AUTHOR("Qumranet");
>  MODULE_LICENSE("GPL");
>
> -static unsigned int halt_poll_ns;
> +/* halt polling only reduces halt latency by 5-7 us, 500us is enough */
> +static unsigned int halt_poll_ns = 500000;
>  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);
> +
> +/* Default resets per-vcpu halt_poll_ns . */
> +static unsigned int halt_poll_ns_shrink;
> +module_param(halt_poll_ns_shrink, int, S_IRUGO);
> +
>  /*
>   * Ordering of locks:
>   *
> @@ -1907,6 +1916,31 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
>  }
>  EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
>
> +static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
> +{
> +       int val = vcpu->halt_poll_ns;
> +
> +       /* 10us base */
> +       if (val == 0 && halt_poll_ns_grow)
> +               val = 10000;
> +       else
> +               val *= halt_poll_ns_grow;
> +
> +       vcpu->halt_poll_ns = val;
> +}
> +
> +static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
> +{
> +       int val = vcpu->halt_poll_ns;
> +
> +       if (halt_poll_ns_shrink == 0)
> +               val = 0;
> +       else
> +               val /= halt_poll_ns_shrink;
> +
> +       vcpu->halt_poll_ns = val;
> +}
> +
>  static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>  {
>         if (kvm_arch_vcpu_runnable(vcpu)) {
> @@ -1929,6 +1963,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>         ktime_t start, cur;
>         DEFINE_WAIT(wait);
>         bool waited = false;
> +       u64 poll_ns = 0, wait_ns = 0, block_ns = 0;
>
>         start = cur = ktime_get();
>         if (vcpu->halt_poll_ns) {
> @@ -1941,10 +1976,15 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>                          */
>                         if (kvm_vcpu_check_block(vcpu) < 0) {
>                                 ++vcpu->stat.halt_successful_poll;
> -                               goto out;
> +                               break;
>                         }
>                         cur = ktime_get();
>                 } while (single_task_running() && ktime_before(cur, stop));
> +
> +               if (ktime_before(cur, stop)) {

You can't use 'cur' to tell if the interrupt arrived. single_task_running()
can break us out of the loop before 'stop'.

> +                       poll_ns = ktime_to_ns(cur) - ktime_to_ns(start);

Put this line before the if(). block_ns should always include the time
spent polling; even if polling does not succeed.

> +                       goto out;
> +               }
>         }
>
>         for (;;) {
> @@ -1959,9 +1999,24 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>
>         finish_wait(&vcpu->wq, &wait);
>         cur = ktime_get();
> +       wait_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>
>  out:
> -       trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
> +       block_ns = poll_ns + wait_ns;
> +
> +       if (halt_poll_ns) {

If you want, you can leave this if() out and save some indentation.

> +               if (block_ns <= vcpu->halt_poll_ns)
> +                       ;
> +               /* we had a long block, shrink polling */
> +               else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> +                       shrink_halt_poll_ns(vcpu);
> +               /* we had a short halt and our poll time is too small */
> +               else if (vcpu->halt_poll_ns < halt_poll_ns &&
> +                       block_ns < halt_poll_ns)
> +                       grow_halt_poll_ns(vcpu);
> +       }
> +
> +       trace_kvm_vcpu_wakeup(block_ns, waited);
>  }
>  EXPORT_SYMBOL_GPL(kvm_vcpu_block);
>
> --
> 1.9.1
>
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