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

[David Matlack]

On Wed, Sep 2, 2015 at 12:12 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
>
> On 02/09/2015 20:09, David Matlack wrote:
>> 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'.
>
> Ah, I thought this was on purpose. :)
>
> If !single_task_running(), it is okay to keep vcpu->halt_poll_ns high,
> because the physical CPU is not going to be idle anyway.  Resetting the
> timer as soon as single_task_running() becomes false will not cost much
> CPU time.

Good point. I agree we can keep halt_poll_ns high in this case.

I actually wasn't thinking about vcpu->halt_poll_ns though. If
single_task_running() breaks us out of the loop we will "goto out" instead
of scheduling. My suspicion is this will cause us to loop calling
kvm_vcpu_block and starve the waiting task (at least until need_resched()),
which would break the "only hog the cpu when idle" aspect of halt-polling.

>
> Does it make sense?
>
> Paolo
>
>>> +                       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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