[SRU][Jammy][lowlatency] Ubuntu: [Config] lowlatency: enhance desktop responsiveness

[Gerald Yang]

BugLink: https://bugs.launchpad.net/bugs/2023007

[Impact]

The lowlatency kernel in Ubuntu is specifically designed to prioritize
high responsiveness, making it ideal for multimedia environments like
DAWs and audio processing platforms, as well as soft real-time
environments.

With the introduction of a real-time kernel, it might be worth
reconsidering the role of the lowlatency kernel and potentially
including it as the default kernel in desktop images, focusing on its
suitability for desktop-oriented usage.

To achieve this, we can enable additional configuration settings and
make it more focused for a low-latency and highly responsive desktop
environment.

Optionally (for the future) provide also an additional user-space
package that would enable specific run-time kernel settings focused at
certain preset workload profiles (e.g, web navigation, gaming, audio
processing, etc.).

[Test case]

Use linux-lowlatency in a desktop environment and measure responsiveness
of interactive applications.

[Fix]

Enable the following additional .config settings to make this kernel
more suitable for a low-latency desktop kernel:

 - CONFIG_NO_HZ_FULL=y: enable access to "Full tickless mode" (shutdown
   clock tick when possible across all the enabled CPUs if they are
   either idle or running 1 task - reduce kernel jitter of running tasks
   due to the periodic clock tick, must be enabled at boot time passing
   `nohz_full=<cpu_list>`)

 - CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y: move RCU
   callbacks from softirq context to kthread context (reduce time spent
   in softirqs with preemption disabled to improve the overall system
   responsiveness, at the cost of introducing a potential performance
   penalty, because RCU callbacks are not processed by kernel threads)

[Regression potential]

   Enabling all these settings can introduce a potential performance
   regression, but the kernel should result more responsive and make
   it more suitable for a desktop/multimedia/gaming/audio processing
   context.

Signed-off-by: Gerald Yang <gerald.yang at canonical.com>
---
 debian.lowlatency/config/annotations | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/debian.lowlatency/config/annotations b/debian.lowlatency/config/annotations
index 694ae26b6745..3e1556d60809 100644
--- a/debian.lowlatency/config/annotations
+++ b/debian.lowlatency/config/annotations
@@ -15,12 +15,20 @@ CONFIG_HZ_250                                   note<'Override default HZ used i
 CONFIG_LATENCYTOP                               policy<{'amd64': 'y', 'arm64': 'y'}>
 CONFIG_LATENCYTOP                               note<'https://lists.ubuntu.com/archives/kernel-team/2014-July/045006.html, LP#1655986'>
 
+CONFIG_NO_HZ_FULL                               policy<{'amd64': 'y', 'arm64': 'y'}>
+CONFIG_NO_HZ_FULL                               note<'Enable access to "Full tickless mode" (LP: #2023007)'>
+
+CONFIG_NO_HZ_IDLE                               policy<{'amd64': 'n', 'arm64': 'n'}>
+CONFIG_NO_HZ_IDLE                               note<'Disabled in favor of CONFIG_NO_HZ_FULL (LP: #2023007)'>
+
 CONFIG_PREEMPT                                  policy<{'amd64': 'y', 'arm64': 'y'}>
 CONFIG_PREEMPT                                  note<'Enable fully preemptible kernel'>
 
 CONFIG_PREEMPT_VOLUNTARY                        policy<{'amd64': 'n', 'arm64': 'n'}>
 CONFIG_PREEMPT_VOLUNTARY                        note<'Disable voluntary preemption model'>
 
+CONFIG_RCU_NOCB_CPU                             policy<{'amd64': 'y', 'arm64': 'y'}>
+CONFIG_RCU_NOCB_CPU                             note<'Move RCU callbacks from softirq context to kthread context (LP: #2023007)'>
 
 # ---- Annotations without notes ----
 
@@ -55,6 +63,8 @@ CONFIG_CEC_PIN                                  policy<{'amd64': 'y', 'arm64': '
 CONFIG_CEC_PIN_ERROR_INJ                        policy<{'amd64': 'n', 'arm64': 'n'}>
 CONFIG_COMEDI_TESTS_EXAMPLE                     policy<{'amd64': 'n', 'arm64': 'm'}>
 CONFIG_COMEDI_TESTS_NI_ROUTES                   policy<{'amd64': 'n', 'arm64': 'm'}>
+CONFIG_CONTEXT_TRACKING                         policy<{'amd64': 'y', 'arm64': 'y'}>
+CONFIG_CONTEXT_TRACKING_FORCE                   policy<{'amd64': 'n', 'arm64': 'n'}>
 CONFIG_DEBUG_PREEMPT                            policy<{'amd64': 'n', 'arm64': 'n'}>
 CONFIG_HZ                                       policy<{'amd64': '1000', 'arm64': '1000'}>
 CONFIG_INLINE_READ_LOCK                         policy<{'arm64': '-'}>
@@ -89,4 +99,7 @@ CONFIG_PREEMPT_RCU                              policy<{'amd64': 'y', 'arm64': '
 CONFIG_PREEMPT_TRACER                           policy<{'amd64': 'n', 'arm64': 'n'}>
 CONFIG_TASKS_RCU                                policy<{'amd64': 'y', 'arm64': 'y'}>
 CONFIG_TEST_DIV64                               policy<{'amd64': 'm', 'arm64': 'm'}>
+CONFIG_TICK_CPU_ACCOUNTING                      policy<{'amd64': '-', 'arm64': '-'}>
 CONFIG_UNINLINE_SPIN_UNLOCK                     policy<{'amd64': 'y', 'arm64': 'y'}>
+CONFIG_VIRT_CPU_ACCOUNTING                      policy<{'amd64': 'y', 'arm64': 'y'}>
+CONFIG_VIRT_CPU_ACCOUNTING_GEN                  policy<{'amd64': 'y', 'arm64': 'y'}>
-- 
2.25.1