Re: [PATCH v2 0/4] KVM: optimize the kvm_vcpu_on_spin

["Longpeng (Mike)" <longpeng2@huawei.com>]

On 2017/8/8 19:25, David Hildenbrand wrote:

> On 08.08.2017 06:05, Longpeng(Mike) wrote:
>> This is a simple optimization for kvm_vcpu_on_spin, the
>> main idea is described in patch-1's commit msg.
>>
>> I did some tests base on the RFC version, the result shows
>> that it can improves the performance slightly.
>>
>> == Geekbench-3.4.1 ==
>> VM1: 	8U,4G, vcpu(0...7) is 1:1 pinned to pcpu(6...11,18,19)
>> 	running Geekbench-3.4.1 *10 truns*
>> VM2/VM3/VM4: configure is the same as VM1
>> 	stress each vcpu usage(seed by top in guest) to 40%
>>
>> The comparison of each testcase's score:
>> (higher is better)
>> 		before		after		improve
>> Inter
>>  single		1176.7		1179.0		0.2%
>>  multi		3459.5		3426.5		-0.9%
>> Float
>>  single		1150.5		1150.9		0.0%
>>  multi		3364.5		3391.9		0.8%
>> Memory(stream)
>>  single		1768.7		1773.1		0.2%
>>  multi		2511.6		2557.2		1.8%
>> Overall
>>  single		1284.2		1286.2		0.2%
>>  multi		3231.4		3238.4		0.2%
>>
>>
>> == kernbench-0.42 ==
>> VM1:    8U,12G, vcpu(0...7) is 1:1 pinned to pcpu(6...11,18,19)
>>         running "kernbench -n 10"
>> VM2/VM3/VM4: configure is the same as VM1
>>         stress each vcpu usage(seed by top in guest) to 40%
>>
>> The comparison of 'Elapsed Time':
>> (sooner is better)
>> 		before		after		improve
>> load -j4	12.762		12.751		0.1%
>> load -j32	9.743		8.955		8.1%
>> load -j		9.688		9.229		4.7%
>>
>>
>> Physical Machine:
>>   Architecture:          x86_64
>>   CPU op-mode(s):        32-bit, 64-bit
>>   Byte Order:            Little Endian
>>   CPU(s):                24
>>   On-line CPU(s) list:   0-23
>>   Thread(s) per core:    2
>>   Core(s) per socket:    6
>>   Socket(s):             2
>>   NUMA node(s):          2
>>   Vendor ID:             GenuineIntel
>>   CPU family:            6
>>   Model:                 45
>>   Model name:            Intel(R) Xeon(R) CPU E5-2640 0 @ 2.50GHz
>>   Stepping:              7
>>   CPU MHz:               2799.902
>>   BogoMIPS:              5004.67
>>   Virtualization:        VT-x
>>   L1d cache:             32K
>>   L1i cache:             32K
>>   L2 cache:              256K
>>   L3 cache:              15360K
>>   NUMA node0 CPU(s):     0-5,12-17
>>   NUMA node1 CPU(s):     6-11,18-23
>>
>> ---
>> Changes since V1:
>>  - split the implementation of s390 & arm. [David]
>>  - refactor the impls according to the suggestion. [Paolo]
>>
>> Changes since RFC:
>>  - only cache result for X86. [David & Cornlia & Paolo]
>>  - add performance numbers. [David]
>>  - impls arm/s390. [Christoffer & David]
>>  - refactor the impls. [me]
>>
>> ---
>> Longpeng(Mike) (4):
>>   KVM: add spinlock optimization framework
>>   KVM: X86: implement the logic for spinlock optimization
>>   KVM: s390: implements the kvm_arch_vcpu_in_kernel()
>>   KVM: arm: implements the kvm_arch_vcpu_in_kernel()
>>
>>  arch/arm/kvm/handle_exit.c      |  2 +-
>>  arch/arm64/kvm/handle_exit.c    |  2 +-
>>  arch/mips/kvm/mips.c            |  6 ++++++
>>  arch/powerpc/kvm/powerpc.c      |  6 ++++++
>>  arch/s390/kvm/diag.c            |  2 +-
>>  arch/s390/kvm/kvm-s390.c        |  6 ++++++
>>  arch/x86/include/asm/kvm_host.h |  5 +++++
>>  arch/x86/kvm/hyperv.c           |  2 +-
>>  arch/x86/kvm/svm.c              | 10 +++++++++-
>>  arch/x86/kvm/vmx.c              | 16 +++++++++++++++-
>>  arch/x86/kvm/x86.c              | 11 +++++++++++
>>  include/linux/kvm_host.h        |  3 ++-
>>  virt/kvm/arm/arm.c              |  5 +++++
>>  virt/kvm/kvm_main.c             |  4 +++-
>>  14 files changed, 72 insertions(+), 8 deletions(-)
>>
> 
> I am curious, is there any architecture that allows to trigger
> kvm_vcpu_on_spin(vcpu); while _not_ in kernel mode?


IIUC, X86/SVM will trap to host due to PAUSE insn no matter the vcpu is in
kernel-mode or user-mode.

> 
> I would have guessed that user space should never be allowed to make cpu
> wide decisions (giving up the CPU to the hypervisor).
> 
> E.g. s390x diag can only be executed from kernel space. VMX PAUSE is
> only valid from kernel space.


X86/VMX has "PAUSE exiting" and "PAUSE-loop exiting"(PLE). KVM only uses PLE,
this is as you said "only valid from kernel space"

However, the "PAUSE exiting" can cause user-mode vcpu exit too.

> 
> I.o.w. do we need a parameter to kvm_vcpu_on_spin(vcpu); at all, or is
> "me_in_kernel" basically always true?
> 


-- 
Regards,
Longpeng(Mike)