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SubjectRe: Re: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi
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On 10/21/21 1:03 PM, Wanpeng Li wrote:
> On Thu, 21 Oct 2021 at 11:05, zhenwei pi <pizhenwei@bytedance.com> wrote:
>>
>>
>> On 10/21/21 4:12 AM, Sean Christopherson wrote:
>>> On Wed, Oct 20, 2021, Wanpeng Li wrote:
>>>> On Wed, 20 Oct 2021 at 20:08, zhenwei pi <pizhenwei@bytedance.com> wrote:
>>>>>
>>>>> Although host side exposes KVM PV SEND IPI feature to guest side,
>>>>> guest should still have a chance to disable it.
>>>>>
>>>>> A typicall case of this parameter:
>>>>> If the host AMD server enables AVIC feature, the flat mode of APIC
>>>>> get better performance in the guest.
>>>>
>>>> Hmm, I didn't find enough valuable information in your posting. We
>>>> observe AMD a lot before.
>>>> https://lore.kernel.org/all/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@mail.gmail.com/T/#u
>>>
>>> I too would like to see numbers. I suspect the answer is going to be that
>>> AVIC performs poorly in CPU overcommit scenarios because of the cost of managing
>>> the tables and handling "failed delivery" exits, but that AVIC does quite well
>>> when vCPUs are pinned 1:1 and IPIs rarely require an exit to the host.
>>>
>>
>> Test env:
>> CPU: AMD EPYC 7642 48-Core Processor
>>
>> Kmod args(enable avic and disable nested):
>> modprobe kvm-amd nested=0 avic=1 npt=1
>>
>> QEMU args(disable x2apic):
>> ... -cpu host,x2apic=off ...
>>
>> Benchmark tool:
>> https://github.com/bytedance/kvm-utils/tree/master/microbenchmark/apic-ipi
>>
>> ~# insmod apic_ipi.ko options=5 && dmesg -c
>>
>> apic_ipi: 1 NUMA node(s)
>> apic_ipi: apic [flat]
>> apic_ipi: apic->send_IPI[default_send_IPI_single+0x0/0x40]
>> apic_ipi: apic->send_IPI_mask[kvm_send_ipi_mask+0x0/0x10]
>> apic_ipi: IPI[kvm_send_ipi_mask] from CPU[0] to CPU[1]
>> apic_ipi: total cycles 375671259, avg 3756
>> apic_ipi: IPI[flat_send_IPI_mask] from CPU[0] to CPU[1]
>> apic_ipi: total cycles 221961822, avg 2219
>>
>>
>> apic->send_IPI_mask[kvm_send_ipi_mask+0x0/0x10]
>> -> This line show current send_IPI_mask is kvm_send_ipi_mask(because
>> of PV SEND IPI FEATURE)
>>
>> apic_ipi: IPI[kvm_send_ipi_mask] from CPU[0] to CPU[1]
>> apic_ipi: total cycles 375671259, avg 3756
>> -->These lines show the average cycles of each kvm_send_ipi_mask: 3756
>>
>> apic_ipi: IPI[flat_send_IPI_mask] from CPU[0] to CPU[1]
>> apic_ipi: total cycles 221961822, avg 2219
>> -->These lines show the average cycles of each flat_send_IPI_mask: 2219
>
> Just single target IPI is not eough.
>
> Wanpeng
>

Benchmark smp_call_function_single
(https://github.com/bytedance/kvm-utils/blob/master/microbenchmark/ipi-bench/ipi_bench.c):

Test env:
CPU: AMD EPYC 7642 48-Core Processor

Kmod args(enable avic and disable nested):
modprobe kvm-amd nested=0 avic=1 npt=1

QEMU args(disable x2apic):
... -cpu host,x2apic=off ...

1> without no-kvm-pvipi:
ipi_bench_single wait[1], CPU0[NODE0] -> CPU1[NODE0], loop = 100000
elapsed = 424945631 cycles, average = 4249 cycles
ipitime = 385246136 cycles, average = 3852 cycles
ipi_bench_single wait[0], CPU0[NODE0] -> CPU1[NODE0], loop = 100000
elapsed = 419057953 cycles, average = 4190 cycles

2> with no-kvm-pvipi:
ipi_bench_single wait[1], CPU0[NODE0] -> CPU1[NODE0], loop = 100000
elapsed = 321756407 cycles, average = 3217 cycles
ipitime = 299433550 cycles, average = 2994 cycles
ipi_bench_single wait[0], CPU0[NODE0] -> CPU1[NODE0], loop = 100000
elapsed = 295382146 cycles, average = 2953 cycles


--
zhenwei pi

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