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Description of problem:
There is latency spike when testing kvm-rt guest with nx_huge_pages=Y( >= 50us). Testing with nx_huge_pages=N, the max latency can be lower 22us.
Version-Release number of selected component (if applicable):
rt-tests-1.5-9.el7.x86_64
qemu-kvm-rhev-2.12.0-41.el7.x86_64
libvirt-4.5.0-31.el7.x86_64
microcode_ctl-2.1-61.el7.x86_64
kernel-rt-3.10.0-1121.rt56.1085.el7.x86_64
tuned-2.11.0-8.el7.noarch
How reproducible:
100%
Steps to Reproduce:
1. Setup RT host
2. Set nx_huge_pages_recovery_ratio=0 in host
# modprobe -r kvm_intel
# modprobe -r kvm
# modprobe kvm nx_huge_pages_recovery_ratio=0
# modprobe kvm_intel
# systool -vm kvm | grep nx_huge_pages
nx_huge_pages_recovery_ratio= "0"
nx_huge_pages = "Y"
2. Boot RT guest with 10vCPUs (2 for housekeeping CPUs and 8 for rt vCPUs)
3. Start 24h cyclictest in guest, max latency can reach 50us.
- Single VM with 8 rt vCPUs(max latency is 50us):
# Min Latencies: 00006 00008 00008 00007 00008 00008 00008 00007
# Avg Latencies: 00008 00008 00008 00008 00008 00008 00008 00008
# Max Latencies: 00044 00050 00020 00047 00022 00021 00026 00041
4. If set nx_huge_pages=N, the max latency of 24h cyclictest in above guest is 22us.
# modprobe -r kvm_intel
# modprobe -r kvm
# modprobe kvm nx_huge_pages_recovery_ratio=0 nx_huge_pages=N
# modprobe kvm_intel
# systool -vm kvm | grep nx_huge_pages
nx_huge_pages_recovery_ratio= "0"
nx_huge_pages = "N"
- Single VM with 8 rt vCPUs(max latency is 22us):
# Min Latencies: 00006 00008 00008 00008 00008 00008 00008 00008
# Avg Latencies: 00008 00008 00008 00008 00008 00008 00008 00008
# Max Latencies: 00019 00019 00019 00022 00019 00020 00019 00020
Actual results:
There is large latency spike when testing with default nx_huge_pages value which is Y now.
Expected results:
There should be no large latency spike when testing with default nx_huge_pages value, we request to set it to be default N.
Additional info:
1. We hit this issue on rhel7.6.z kernel-rt-3.10.0-957.42.1.rt56.955.el7.x86_64.
- Single VM with 8 rt vCPUs(max latency is 99us):
# Min Latencies: 00006 00007 00007 00007 00007 00007 00007 00007
# Avg Latencies: 00008 00007 00007 00007 00007 00007 00007 00007
# Max Latencies: 00084 00082 00098 00099 00085 00033 00027 00019
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (Important: kernel-rt security and bug fix update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHSA-2020:4062
Description of problem: There is latency spike when testing kvm-rt guest with nx_huge_pages=Y( >= 50us). Testing with nx_huge_pages=N, the max latency can be lower 22us. Version-Release number of selected component (if applicable): rt-tests-1.5-9.el7.x86_64 qemu-kvm-rhev-2.12.0-41.el7.x86_64 libvirt-4.5.0-31.el7.x86_64 microcode_ctl-2.1-61.el7.x86_64 kernel-rt-3.10.0-1121.rt56.1085.el7.x86_64 tuned-2.11.0-8.el7.noarch How reproducible: 100% Steps to Reproduce: 1. Setup RT host 2. Set nx_huge_pages_recovery_ratio=0 in host # modprobe -r kvm_intel # modprobe -r kvm # modprobe kvm nx_huge_pages_recovery_ratio=0 # modprobe kvm_intel # systool -vm kvm | grep nx_huge_pages nx_huge_pages_recovery_ratio= "0" nx_huge_pages = "Y" 2. Boot RT guest with 10vCPUs (2 for housekeeping CPUs and 8 for rt vCPUs) 3. Start 24h cyclictest in guest, max latency can reach 50us. - Single VM with 8 rt vCPUs(max latency is 50us): # Min Latencies: 00006 00008 00008 00007 00008 00008 00008 00007 # Avg Latencies: 00008 00008 00008 00008 00008 00008 00008 00008 # Max Latencies: 00044 00050 00020 00047 00022 00021 00026 00041 4. If set nx_huge_pages=N, the max latency of 24h cyclictest in above guest is 22us. # modprobe -r kvm_intel # modprobe -r kvm # modprobe kvm nx_huge_pages_recovery_ratio=0 nx_huge_pages=N # modprobe kvm_intel # systool -vm kvm | grep nx_huge_pages nx_huge_pages_recovery_ratio= "0" nx_huge_pages = "N" - Single VM with 8 rt vCPUs(max latency is 22us): # Min Latencies: 00006 00008 00008 00008 00008 00008 00008 00008 # Avg Latencies: 00008 00008 00008 00008 00008 00008 00008 00008 # Max Latencies: 00019 00019 00019 00022 00019 00020 00019 00020 Actual results: There is large latency spike when testing with default nx_huge_pages value which is Y now. Expected results: There should be no large latency spike when testing with default nx_huge_pages value, we request to set it to be default N. Additional info: 1. We hit this issue on rhel7.6.z kernel-rt-3.10.0-957.42.1.rt56.955.el7.x86_64. - Single VM with 8 rt vCPUs(max latency is 99us): # Min Latencies: 00006 00007 00007 00007 00007 00007 00007 00007 # Avg Latencies: 00008 00007 00007 00007 00007 00007 00007 00007 # Max Latencies: 00084 00082 00098 00099 00085 00033 00027 00019