[PATCH v2 0/5] KVM: X86: Scaling Guest OS Critical Sections with boosting

Wanpeng Li
2022-04-01T08:11:33Z

The missing semantic gap that occurs when a guest OS is preempted
when executing its own critical section, this leads to degradation
of application scalability. We try to bridge this semantic gap in
some ways, by passing guest preempt_count to the host and checking
guest irq disable state, the hypervisor now knows whether guest
OSes are running in the critical section, the hypervisor yield-on-spin
heuristics can be more smart this time to boost the vCPU candidate
who is in the critical section to mitigate this preemption problem,
in addition, it is more likely to be a potential lock holder.

Testing on 96 HT 2 socket Xeon CLX server, with 96 vCPUs VM 100GB RAM,
one VM running benchmark, the other(none-2) VMs running cpu-bound
workloads, There is no performance regression for other benchmarks
like Unixbench etc.

1VM
vanilla optimized improved

hackbench -l 50000
28 21.45 30.5%
ebizzy -M
12189 12354 1.4%
dbench
712 MB/sec 722 MB/sec 1.4%

2VM:
vanilla optimized improved

hackbench -l 10000
29.4 26 13%
ebizzy -M
3834 4033 5%
dbench
42.3 MB/sec 44.1 MB/sec 4.3%

3VM:
vanilla optimized improved

hackbench -l 10000
47 35.46 33%
ebizzy -M
3828 4031 5%
dbench
30.5 MB/sec 31.16 MB/sec 2.3%

v1 -> v2:
* add more comments to irq disable state
* renaming irq_disabled to last_guest_irq_disabled
* renaming, inverting the return, and also return a bool for kvm_vcpu_non_preemptable

Wanpeng Li (5):
KVM: X86: Add MSR_KVM_PREEMPT_COUNT support
KVM: X86: Add last guest interrupt disable state support
KVM: X86: Boost vCPU which is in critical section
x86/kvm: Add MSR_KVM_PREEMPT_COUNT guest support
KVM: X86: Expose PREEMT_COUNT CPUID feature bit to guest

Documentation/virt/kvm/cpuid.rst | 3 ++
arch/x86/include/asm/kvm_host.h | 8 ++++
arch/x86/include/uapi/asm/kvm_para.h | 2 +
arch/x86/kernel/kvm.c | 10 +++++
arch/x86/kvm/cpuid.c | 3 +-
arch/x86/kvm/x86.c | 60 ++++++++++++++++++++++++++++
include/linux/kvm_host.h | 1 +
virt/kvm/kvm_main.c | 7 ++++
8 files changed, 93 insertions(+), 1 deletion(-)

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2.25.1