From baefc3b4643d9e61361cacf46dfbeb95ce96933b Mon Sep 17 00:00:00 2001
From: Tom Weininger <tweining@redhat.com>
Date: Fri, 19 Aug 2022 11:00:31 +0200
Subject: [PATCH] Set nohz_full in cpu-pinning element

nohz_full (or full dynticks) is a Linux kernel setting that should reduce
kernel noise on configured CPUs to a minimum. This change activates this
setting on all vCPUs except the first one.

Change-Id: Ie009aff52d572873da0bb5cc43fedd7f3325dccc
---
 .../environment.d/80-kernel-cpu-affinity            |  1 +
 ...hz-full-to-improve-latency-9d5acd7333f7e462.yaml | 13 +++++++++++++
 2 files changed, 14 insertions(+)
 create mode 100644 releasenotes/notes/use-nohz-full-to-improve-latency-9d5acd7333f7e462.yaml

diff --git a/elements/cpu-pinning/environment.d/80-kernel-cpu-affinity b/elements/cpu-pinning/environment.d/80-kernel-cpu-affinity
index 198aef0bca..76bcf2f55e 100644
--- a/elements/cpu-pinning/environment.d/80-kernel-cpu-affinity
+++ b/elements/cpu-pinning/environment.d/80-kernel-cpu-affinity
@@ -27,6 +27,7 @@ case $DISTRO_NAME in
         DIB_BOOTLOADER_DEFAULT_CMDLINE+=" irqaffinity=0"
         # This will be ignored on single vCPU systems
         DIB_BOOTLOADER_DEFAULT_CMDLINE+=" isolcpus=1-N"
+        DIB_BOOTLOADER_DEFAULT_CMDLINE+=" nohz=on nohz_full=1-N"
         export DIB_BOOTLOADER_DEFAULT_CMDLINE
     ;;
     *)
diff --git a/releasenotes/notes/use-nohz-full-to-improve-latency-9d5acd7333f7e462.yaml b/releasenotes/notes/use-nohz-full-to-improve-latency-9d5acd7333f7e462.yaml
new file mode 100644
index 0000000000..b3dfa23c5a
--- /dev/null
+++ b/releasenotes/notes/use-nohz-full-to-improve-latency-9d5acd7333f7e462.yaml
@@ -0,0 +1,13 @@
+---
+features:
+  - |
+    The cpu-pinning element for the amphora image sets the kernel bootarg
+    nohz_full=1-N to enable
+    full dynticks on all CPUs except the first one (on single CPU images this
+    will have no effect). This should reduce kernel noise on those CPUs to a
+    minimum and reduce latency.
+upgrade:
+  - |
+    In order for the full dynticks optimization to become effective a new 
+    amphora image needs to be built with the new optional CPU pinning 
+    feature enabled.