Disable DEBUG_PREEMPT in jammy and noble realtime kernels

[Kevin Becker]

SRU Justification:

[Impact]

CONFIG_DEBUG_PREEMPT was accidentally enabled on jammy last year as part of 5.15.0-1040.45 when we migrated to annotations for kernel configs. It was also copied into the noble realtime kernel. This config can introduce additional latency. More info on this config is at https://cateee.net/lkddb/web-lkddb/DEBUG_PREEMPT.html

[Fix]

CONFIG_DEBUG_PREEMPT should be disabled on both jammy and noble.

[Test Plan]

We will use our kernel regression testing to confirm this doesn't cause any regressions. This will include cyclictest and other latency tests.

[Where problems could occur]

This change is limited. If code elsewhere inadvertently depends on this additional latency, it could cause a problem.

[Kevin Becker (kevinbecker)]

This change has been tested and shows improvement in latencies with cyclictest, even with an untuned system.

Jammy 5.15:

1066 (PREEMPT_DEBUG enabled):

17:18:55 INFO | START rt_tests_cyclictest.rt_tests_cyclictest rt_tests_cyclictest.rt_tests_cyclictest timestamp=1721755135 localtime=Jul 23 17:18:55
17:18:55 DEBUG| Persistent state client._record_indent now set to 2
17:18:55 DEBUG| Persistent state client.unexpected_reboot now set to ('rt_tests_cyclictest.rt_tests_cyclictest', 'rt_tests_cyclictest.rt_tests_cyclictest')
17:18:55 WARNI| System python is too old, crash handling disabled
17:18:55 DEBUG| Running '/home/ubuntu/autotest/client/tmp/rt_tests_cyclictest/src/rt-tests/cyclictest -t 10 -m -l 100000 -p 80 -i 200 -d 0 -b 200'
17:18:55 DEBUG| [stdout] # /dev/cpu_dma_latency set to 0us
17:18:55 DEBUG| [stdout] policy: fifo: loadavg: 14.85 3.71 1.35 1/772 3846
17:19:15 DEBUG| [stdout]
17:19:15 DEBUG| [stdout] T: 0 ( 3837) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 22
17:19:15 DEBUG| [stdout] T: 1 ( 3838) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 11
17:19:15 DEBUG| [stdout] T: 2 ( 3839) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 19
17:19:15 DEBUG| [stdout] T: 3 ( 3840) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 22
17:19:15 DEBUG| [stdout] T: 4 ( 3841) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 47
17:19:15 DEBUG| [stdout] T: 5 ( 3842) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 20
17:19:15 DEBUG| [stdout] T: 6 ( 3843) P:80 I:200 C: 100000 Min: 2 Act: 2 Avg: 2 Max: 8
17:19:15 DEBUG| [stdout] T: 7 ( 3844) P:80 I:200 C: 99995 Min: 2 Act: 2 Avg: 2 Max: 55
17:19:15 DEBUG| [stdout] T: 8 ( 3845) P:80 I:200 C: 99993 Min: 2 Act: 2 Avg: 2 Max: 18
17:19:15 DEBUG| [stdout] T: 9 ( 3846) P:80 I:200 C: 99984 Min: 2 Act: 2 Avg: 2 Max: 15
17:19:15 DEBUG| [stdout] # Thread Ids: 03837 03838 03839 03840 03841 03842 03843 03844 03845 03846
17:19:15 INFO | GOOD rt_tests_cyclictest.rt_tests_cyclictest rt_tests_cyclictest.rt_tests_cyclictest timestamp=1721755155 localtime=Jul 23 17:19:15 completed successfully
17:19:15 INFO | END GOOD rt_tests_cyclictest.rt_tests_cyclictest rt_tests_cyclictest.rt_tests_cyclictest timestamp=1721755155 localtime=Jul 23 17:19:15
17:19:15 DEBUG| Persistent state client._record_indent now set to 1
17:19:15 DEBUG| Persistent state client.unexpected_reboot deleted
17:19:15 INFO | END GOOD ---- ---- timestamp=1721755155 localtime=Jul 23 17:19:15
17:19:15 DEBUG| Persistent state client._record_indent now set to 0
17:19:15 INFO | Report successfully generated at /home/ubuntu/autotest/client/results/default/job_report.html

1068 (PREEMPT_DEBUG disabled):

12:52:04 INFO | START rt_tests_cyclictest.rt_tests_cyclictest rt_tests_cyclictest.rt_tests_cyclictest timestamp=1721825524 localtime=Jul 24 12:52:04
12:52:04 DEBUG| Persistent state client._record_indent now set to 2
12:52:04 DEBUG| Persistent state client.unexpected_reboot now set to ('rt_tests_cyclictest.rt_tests_cyclictest', 'rt_tests_cyclictest.rt_tests_cyclictest')
12:52:04 WARNI| Sy...

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-realtime/6.8.1-1006.6 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-noble-linux-realtime' to 'verification-done-noble-linux-realtime'. If the problem still exists, change the tag 'verification-needed-noble-linux-realtime' to 'verification-failed-noble-linux-realtime'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-realtime/5.15.0-1068.76 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-jammy-linux-realtime' to 'verification-done-jammy-linux-realtime'. If the problem still exists, change the tag 'verification-needed-jammy-linux-realtime' to 'verification-failed-jammy-linux-realtime'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Kevin Becker (kevinbecker)]

Testing passes for jammy:linux-realtime/5.15.0-1068.76 and noble:linux-realtime/6.8.1-1006.6.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-nvidia-tegra/5.15.0-1027.27 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-jammy-linux-nvidia-tegra' to 'verification-done-jammy-linux-nvidia-tegra'. If the problem still exists, change the tag 'verification-needed-jammy-linux-nvidia-tegra' to 'verification-failed-jammy-linux-nvidia-tegra'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-nvidia-tegra-igx/5.15.0-1015.15 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-jammy-linux-nvidia-tegra-igx' to 'verification-done-jammy-linux-nvidia-tegra-igx'. If the problem still exists, change the tag 'verification-needed-jammy-linux-nvidia-tegra-igx' to 'verification-failed-jammy-linux-nvidia-tegra-igx'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-raspi-realtime/6.8.0-2008.8 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-noble-linux-raspi-realtime' to 'verification-done-noble-linux-raspi-realtime'. If the problem still exists, change the tag 'verification-needed-noble-linux-raspi-realtime' to 'verification-failed-noble-linux-raspi-realtime'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Kevin Becker (kevinbecker)]

This was fixed in 6.8.1-1006.6 realtime on Noble and 5.15.0-1068.76 realtime on Jammy.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-nvidia-tegra-5.15/5.15.0-1027.27~20.04.1 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-focal-linux-nvidia-tegra-5.15' to 'verification-done-focal-linux-nvidia-tegra-5.15'. If the problem still exists, change the tag 'verification-needed-focal-linux-nvidia-tegra-5.15' to 'verification-failed-focal-linux-nvidia-tegra-5.15'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!