linux-kernel - Re: Re: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi

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Date:   Thu, 21 Oct 2021 15:17:21 +0800
From:   zhenwei pi <pizhenwei@...edance.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org
Subject: Re: Re: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi

On 10/21/21 1:03 PM, Wanpeng Li wrote:
> On Thu, 21 Oct 2021 at 11:05, zhenwei pi <pizhenwei@...edance.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@...edance.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