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Bug 1772894 - kvm nx_huge_pages_recovery_ratio=0 is needed to meet KVM-RT low latency requirement
Summary: kvm nx_huge_pages_recovery_ratio=0 is needed to meet KVM-RT low latency requi...
Keywords:
Status: CLOSED ERRATA
Alias: None
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: kernel-rt
Version: 7.8
Hardware: Unspecified
OS: Unspecified
high
high
Target Milestone: rc
: ---
Assignee: Tom Rix
QA Contact: Pei Zhang
URL:
Whiteboard:
: 1772651 1779455 (view as bug list)
Depends On: 1772738
Blocks: 1672377 1715542 1779458 1781156 1781157
TreeView+ depends on / blocked
 
Reported: 2019-11-15 13:19 UTC by Paolo Bonzini
Modified: 2020-03-31 19:52 UTC (History)
16 users (show)

Fixed In Version:
Doc Type: If docs needed, set a value
Doc Text:
Clone Of: 1772738
: 1781156 1781157 (view as bug list)
Environment:
Last Closed: 2020-03-31 19:51:08 UTC
Target Upstream Version:
Embargoed:

Attachments (Terms of Use)


Links
System ID Private Priority Status Summary Last Updated
Red Hat Product Errata RHSA-2020:1070 0 None None None 2020-03-31 19:52:55 UTC

Comment 2 Beth Uptagrafft 2019-11-15 15:57:59 UTC 
*** Bug 1772651 has been marked as a duplicate of this bug. ***
Comment 10 Clark Williams 2019-12-04 20:33:06 UTC 
*** Bug 1779455 has been marked as a duplicate of this bug. ***
Comment 14 Paolo Bonzini 2019-12-05 00:25:15 UTC 
The mitigation itself does not cause jitter, it only slows down the guest somewhat. Real-time is not about running things fast, but "fast enough" and predictably.
Comment 15 Peter Xu 2019-12-05 01:06:37 UTC 
(In reply to Paolo Bonzini from comment #14)
> The mitigation itself does not cause jitter, it only slows down the guest
> somewhat. Real-time is not about running things fast, but "fast enough" and
> predictably.

Makes sense.  Sorry I confused with this - of course we can't turn off a mitigation by default.

Then we probably need to add kvm.nx_huge_pages=0 manually when testing with all mitigations=off case for kvm-rt, because IIUC otherwise the guest could stop to use huge pages on some page table entries witout notice.
Comment 16 Paolo Bonzini 2019-12-06 14:42:24 UTC 
Mitigations=off already disables nx_huge_pages=0 so we're safe for that.
Comment 21 Pei Zhang 2020-01-03 09:08:32 UTC 
*** Bug 1779455 has been marked as a duplicate of this bug. ***
Comment 25 Pei Zhang 2020-02-04 01:17:11 UTC 
Verified with kernel-rt-3.10.0-1125.rt56.1091.el7.x86_64:

1. Default nx_huge_pages_recovery_ratio is 0. This is expected.
# systool -vm kvm | grep nx_hug
    nx_huge_pages_recovery_ratio= "0"
    nx_huge_pages       = "Y"


2. KVM-RT acceptance get PASS. 1 hour cyclictest max latency is 21us.

==Results==
(1)Single VM with 1 rt vCPU:
# Min Latencies: 00006
# Avg Latencies: 00007
# Max Latencies: 00020

(2)Single VM with 8 rt vCPUs:
# Min Latencies: 00006 00008 00008 00008 00008 00008 00008 00008
# Avg Latencies: 00007 00008 00008 00008 00008 00008 00008 00008
# Max Latencies: 00021 00020 00019 00018 00018 00019 00019 00018

(3)Multiple VMs each with 1 rt vCPU:
- VM1
# Min Latencies: 00006
# Avg Latencies: 00007
# Max Latencies: 00020

- VM2
# Min Latencies: 00006
# Avg Latencies: 00007
# Max Latencies: 00018

- VM3
# Min Latencies: 00006
# Avg Latencies: 00007
# Max Latencies: 00019

- VM4
# Min Latencies: 00005
# Avg Latencies: 00007
# Max Latencies: 00018



==Versions==
qemu-kvm-tools-rhev-2.12.0-43.el7.x86_64
microcode_ctl-2.1-61.el7.x86_64
rt-tests-1.5-9.el7.x86_64
kernel-rt-3.10.0-1125.rt56.1091.el7.x86_64
qemu-kvm-rhev-2.12.0-43.el7.x86_64
qemu-kvm-common-rhev-2.12.0-43.el7.x86_64
tuned-2.11.0-8.el7.noarch
libvirt-4.5.0-32.el7.x86_64


Beaker job:https://beaker.engineering.redhat.com/jobs/4048311

So this bug has been fixed very well. Move to 'VERIFIED'.
Comment 27 errata-xmlrpc 2020-03-31 19:51:08 UTC 
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, 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-2020:1070
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