Bug 15252 - Enables CONFIG_DEBUG_PREEMPT
Summary: Enables CONFIG_DEBUG_PREEMPT
Status: RESOLVED FIXED
Alias: None
Product: OE-Core
Classification: Build System, Metadata & Runtime
Component: kernel (show other bugs)
Version: 4.3
Hardware: x86 Multiple
: Medium+ normal
Target Milestone: 5.1
Assignee: Bruce Ashfield
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Reported: 2023-10-23 11:35 UTC by Ross Burton
Modified: 2024-03-25 23:05 UTC (History)
2 users (show)

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OS type for building Yocto: ---
Type of Regression: ---
Verified:
Documentation change: No (bug/feature does not impact docs)

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Description Ross Burton 2023-10-23 11:35:01 UTC 
Our kernel enables CONFIG_DEBUG_PREEMPT:

$ grep CONFIG_DEBUG_PREEMPT /yocto/ross/build/tmp/work/qemuarm64-poky-linux/linux-yocto/6.5.7+git/linux-qemuarm64-standard-build/.config
CONFIG_DEBUG_PREEMPT=y

(poky nanbield, qemuarm64)

As per https://lore.kernel.org/lkml/ZTKScoO9kz_LxNMG@ariel/ this is basically a 20% performance hit in some benchmarks.  I think we should disable it in default kernels.
Comment 1 Bruce Ashfield 2023-10-23 15:01:05 UTC 
We don't have it on by default, only the selftests do:

lib/oeqa/selftest/cases/runtime_test.py:KERNEL_EXTRA_FEATURES:append = " features/debug/debug-kernel.scc"


So if we don't want it on for the tests, we need to change it there .. but that would get us less information about crashes and issues, so it isn't my call to make.
Comment 2 Ross Burton 2023-10-23 15:36:59 UTC 
I hate to disagree with you, but in a brand new build tree:

$ bitbake virtual/kernel -c showconfig
Config file written to /yocto/ross/buildnew/tmp/work/qemux86_64-poky-linux/linux-yocto/6.5.7+git/linux-qemux86_64-standard-build/.config

$ grep DEBUG.*= /yocto/ross/buildnew/tmp/work/qemux86_64-poky-linux/linux-yocto/6.5.7+git/linux-qemux86_64-standard-build/.config
CONFIG_CGROUP_DEBUG=y
CONFIG_X86_DEBUGCTLMSR=y
CONFIG_PM_DEBUG=y
CONFIG_PM_ADVANCED_DEBUG=y
CONFIG_PM_SLEEP_DEBUG=y
CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC=y
CONFIG_BLK_DEBUG_FS=y
CONFIG_BT_DEBUGFS=y
CONFIG_PNP_DEBUG_MESSAGES=y
CONFIG_SCSI_DEBUG=m
CONFIG_CIFS_DEBUG=y
CONFIG_DEBUG_BUGVERBOSE=y
CONFIG_DEBUG_KERNEL=y
CONFIG_DEBUG_MISC=y
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
CONFIG_DEBUG_INFO_COMPRESSED_NONE=y
CONFIG_DEBUG_FS=y
CONFIG_DEBUG_FS_ALLOW_ALL=y
CONFIG_ARCH_HAS_EARLY_DEBUG=y
CONFIG_SLUB_DEBUG=y
CONFIG_ARCH_HAS_DEBUG_WX=y
CONFIG_HAVE_DEBUG_KMEMLEAK=y
CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE=y
CONFIG_ARCH_HAS_DEBUG_VIRTUAL=y
CONFIG_DEBUG_MEMORY_INIT=y
CONFIG_SCHED_DEBUG=y
CONFIG_DEBUG_PREEMPT=y
CONFIG_LOCK_DEBUGGING_SUPPORT=y
CONFIG_X86_DEBUG_FPU=y

After the configure task the top-level .config is the active config, correct?
Comment 3 Bruce Ashfield 2023-10-23 17:00:31 UTC 
Right, it is also enabled by the poky distro:

meta-poky/conf/distro/poky.conf:KERNEL_EXTRA_FEATURES:append = " features/debug/debug-kernel.scc"

So that is your culprit for the enable.
Comment 4 Ross Burton 2023-10-23 17:01:55 UTC 
That seems suboptimal.
Comment 5 Ross Burton 2023-10-23 17:02:48 UTC 
If SPDX needs symbols, can we have separate "debug symbols" and "debug statements"?
Comment 6 Bruce Ashfield 2023-11-09 15:36:28 UTC 
This is under unit test now.