RE: [PATCH v2] x86/hyper-v: micro-optimize send_ipi_one case

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Michael Kelley <mikelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com>
>> 
>> When sending an IPI to a single CPU there is no need to deal with cpumasks.
>> With 2 CPU guest on WS2019 I'm seeing a minor (like 3%, 8043 -> 7761 CPU
>> cycles) improvement with smp_call_function_single() loop benchmark. The
>> optimization, however, is tiny and straitforward. Also, send_ipi_one() is
>> important for PV spinlock kick.
>> 
>> I was also wondering if it would make sense to switch to using regular
>> APIC IPI send for CPU > 64 case but no, it is twice as expesive (12650 CPU
>> cycles for __send_ipi_mask_ex() call, 26000 for orig_apic.send_IPI(cpu,
>> vector)).
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> Changes since v1:
>>  - Style changes [Roman, Joe]
>> ---
>>  arch/x86/hyperv/hv_apic.c           | 13 ++++++++++---
>>  arch/x86/include/asm/trace/hyperv.h | 15 +++++++++++++++
>>  2 files changed, 25 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
>> index e01078e93dd3..fd17c6341737 100644
>> --- a/arch/x86/hyperv/hv_apic.c
>> +++ b/arch/x86/hyperv/hv_apic.c
>> @@ -194,10 +194,17 @@ static bool __send_ipi_mask(const struct cpumask *mask, int
>> vector)
>> 
>>  static bool __send_ipi_one(int cpu, int vector)
>>  {
>> -	struct cpumask mask = CPU_MASK_NONE;
>> +	trace_hyperv_send_ipi_one(cpu, vector);
>> 
>> -	cpumask_set_cpu(cpu, &mask);
>> -	return __send_ipi_mask(&mask, vector);
>> +	if (!hv_hypercall_pg || (vector < HV_IPI_LOW_VECTOR) ||
>> +	    (vector > HV_IPI_HIGH_VECTOR))
>> +		return false;
>> +
>> +	if (cpu >= 64)
>> +		return __send_ipi_mask_ex(cpumask_of(cpu), vector);
>
> The above test should be checking the VP number, not the CPU
> number,

Oops, of course, thanks for catching this! v3 is coming!

>  since the VP number is used to form the bitmap argument
> to the hypercall.  In all current implementations of Hyper-V, the CPU number
> and VP number are the same as far as I am aware, but that's not guaranteed in 
> the future.
>
> Michael
>
>> +
>> +	return !hv_do_fast_hypercall16(HVCALL_SEND_IPI, vector,
>> +			       BIT_ULL(hv_cpu_number_to_vp_number(cpu)));
>>  }
>> 

-- 
Vitaly