DOC: BMC KVM access will cause large cyclictest latency

[Elisamara Aoki Gonçalves]

Brief Description
-----------------
The customer sees if there are inputs or outputs on BMC KVM, the cyclictest running on isolated cores will be impacted, its latency will increase from about 3us to over 100us like below.

/dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 6.22 7.22 5.47 1/2234 47
T: 0 ( 32) P:99 I:1000 C: 966893 Min: 1 Act: 1 Avg: 1 Max: 101
T: 1 ( 33) P:99 I:1000 C: 966894 Min: 1 Act: 1 Avg: 1 Max: 120
T: 2 ( 34) P:99 I:1000 C: 966893 Min: 1 Act: 1 Avg: 1 Max: 118
T: 3 ( 35) P:99 I:1000 C: 966886 Min: 1 Act: 1 Avg: 1 Max: 142
T: 4 ( 36) P:99 I:1000 C: 966883 Min: 1 Act: 1 Avg: 1 Max: 124
T: 5 ( 37) P:99 I:1000 C: 966883 Min: 1 Act: 1 Avg: 1 Max: 192
T: 6 ( 38) P:99 I:1000 C: 966880 Min: 1 Act: 1 Avg: 1 Max: 73
T: 7 ( 39) P:99 I:1000 C: 966874 Min: 1 Act: 1 Avg: 1 Max: 154
T: 8 ( 40) P:99 I:1000 C: 966866 Min: 1 Act: 1 Avg: 1 Max: 132
T: 9 ( 41) P:99 I:1000 C: 966868 Min: 1 Act: 1 Avg: 1 Max: 196
T:10 ( 42) P:99 I:1000 C: 966866 Min: 1 Act: 1 Avg: 1 Max: 210
T:11 ( 43) P:99 I:1000 C: 966859 Min: 1 Act: 1 Avg: 1 Max: 181
T:12 ( 44) P:99 I:1000 C: 966846 Min: 1 Act: 1 Avg: 1 Max: 106
T:13 ( 45) P:99 I:1000 C: 966848 Min: 1 Act: 1 Avg: 1 Max: 127
T:14 ( 46) P:99 I:1000 C: 966849 Min: 1 Act: 1 Avg: 1 Max: 108
T:15 ( 47) P:99 I:1000 C: 966846 Min: 1 Act: 1 Avg: 1 Max: 119

Severity
--------
<Minor: System/Feature is usable with minor issue>

Steps to Reproduce
------------------
Need add the info "If using the lowlat kernel, the customer should be doing a serial console install, not a graphics console install." in our internal document.

Graphic console will impact rt performance.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/docs/+/882751
Committed: https://opendev.org/starlingx/docs/commit/8e0d4f6df5c09b2a89c23a609d9ecde2a3dfea6b
Submitter: "Zuul (22348)"
Branch: master

commit 8e0d4f6df5c09b2a89c23a609d9ecde2a3dfea6b
Author: Elaine Fonaro <email address hidden>
Date: Tue May 9 16:36:12 2023 -0300

    BMC KVM access will cause large cyclictest latency (r8, dsr8, r7, dsr7)

    - Added a note for low latency kernel.
    - Updated the note to a warning.

    closes-bug: 2019120

    Signed-off-by: Elaine Fonaro <email address hidden>
    Change-Id: I9855b366f084a5d9dd5f8b1697cfe1c0d25850c0

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: r/stx.8.0
Review: https://review.opendev.org/c/starlingx/docs/+/882779

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: r/stx.7.0
Review: https://review.opendev.org/c/starlingx/docs/+/883012

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/docs/+/882779
Committed: https://opendev.org/starlingx/docs/commit/ec4fd21a553b96f5a01c2ad93ed27eac2ae4a788
Submitter: "Zuul (22348)"
Branch: r/stx.8.0

commit ec4fd21a553b96f5a01c2ad93ed27eac2ae4a788
Author: Elaine Fonaro <email address hidden>
Date: Tue May 9 16:36:12 2023 -0300

    BMC KVM access will cause large cyclictest latency (r8, dsr8, r7, dsr7)

    - Added a note for low latency kernel.
    - Updated the note to a warning.

    closes-bug: 2019120

    Signed-off-by: Elaine Fonaro <email address hidden>
    Change-Id: I9855b366f084a5d9dd5f8b1697cfe1c0d25850c0
    (cherry picked from commit 8e0d4f6df5c09b2a89c23a609d9ecde2a3dfea6b)

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/docs/+/883012
Committed: https://opendev.org/starlingx/docs/commit/d9ec1b8fb51e6c93acafdec616aeac66920a0dc9
Submitter: "Zuul (22348)"
Branch: r/stx.7.0

commit d9ec1b8fb51e6c93acafdec616aeac66920a0dc9
Author: Elaine Fonaro <email address hidden>
Date: Tue May 9 16:36:12 2023 -0300

    BMC KVM access will cause large cyclictest latency (r8, dsr8, r7, dsr7)

    - Added a note for low latency kernel.
    - Updated the note to a warning.

    closes-bug: 2019120

    Signed-off-by: Elaine Fonaro <email address hidden>
    Change-Id: I9855b366f084a5d9dd5f8b1697cfe1c0d25850c0
    (cherry picked from commit 8e0d4f6df5c09b2a89c23a609d9ecde2a3dfea6b)