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Bug 1772653

Summary: Latency spikes after recent CVE application
Product: Red Hat Enterprise Linux 8 Reporter: Tom Rix <trix>
Component: kernel-rtAssignee: Clark Williams <williams>
kernel-rt sub component: Memory Management QA Contact: Qiao Zhao <qzhao>
Status: CLOSED DUPLICATE Docs Contact:
Severity: unspecified    
Priority: unspecified CC: bhu, knoel, mm-maint, pbonzini, qzhao, rt-maint, rt-qe
Version: 8.2   
Target Milestone: rc   
Target Release: 8.0   
Hardware: Unspecified   
OS: Unspecified   
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Fixed In Version: Doc Type: If docs needed, set a value
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Last Closed: 2019-11-15 13:16:14 UTC Type: Bug
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description Tom Rix 2019-11-14 19:52:42 UTC 
Description of problem:


The KVM-RT team saw large cyclictest latency's during testing of RT kernels
that were built with the recent set of CVEs.

One of these CVEs, CVE-2018-1220, introduced code for recovering
shattered huge pages in the kvm/mmu.c code. A settable
variable in this code, nx_huge_pages_recovery_ratio, was defaulted to
the value 60, which means that the recovery code would loop potentially
60 times while the kvm->mmu_lock was held. This bit of code introduced
very large (> 5000us) on single VMs with multiple vCPUs. On a single VM with 1
vCPU, the max latency was around 138us which is still unacceptable. 

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Comment 3 Paolo Bonzini 2019-11-15 13:16:14 UTC 
This is not related to the JCC erratum, so I'm closing this as a duplicate of bug 1772738, which points to the right fix.

*** This bug has been marked as a duplicate of bug 1772738 ***