Bug 1772651

Summary: Latency spikes after recent CVE application
Product: Red Hat Enterprise Linux 7 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, mm-maint, qzhao, rt-maint, rt-qe
Version: 7.8   
Target Milestone: rc   
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Hardware: Unspecified   
OS: Unspecified   
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Last Closed: 2019-11-15 15:57:59 UTC Type: Bug
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Description Tom Rix 2019-11-14 19:47:16 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 Beth Uptagrafft 2019-11-15 15:57:59 UTC 

*** This bug has been marked as a duplicate of bug 1772894 ***