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

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/09/2015 20:09, David Matlack wrote:
> On Wed, Sep 2, 2015 at 12:29 AM, Wanpeng Li <wanpeng.li@hotmail.com> 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@google.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
>> ---
>>  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.

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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