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To report a Linux kernel regression, show that the same application or practical use case worked on an older kernel and fails or works worse on a newer one built with a similar configuration. Put [REGRESSION] at the start of the email subject, copy [email protected], give the last known-good and first affected kernel versions, and add a regzbot tracking command. If you can reproduce the issue reliably, kernel bisection can help identify the change that caused it.
What counts as a kernel regression?
The Linux kernel documentation defines a regression as an application or practical use case that worked with one kernel but works worse or not at all with a newer kernel compiled using a similar configuration. The boundary matters: this definition concerns interfaces the kernel provides to userland, not kernel-internal interfaces used by externally developed modules. A small benchmark fluctuation alone is not a meaningful performance or power regression; the change should be significant.
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Configuration differences can make a comparison misleading. When building kernels yourself, use the older kernel’s configuration as a base for the newer one and run make olddefconfig. When testing precompiled kernels, ask the kernel provider about configuration differences if you are unsure whether the builds are comparable. The kernel reporting guide explains this comparison standard.
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Confirm the change is new
Find a kernel version on which the same application or use case worked, then confirm that the issue appears on a newer version. Record the last version that worked and the first one that failed; “it broke after an update” is not precise enough to guide an investigation.
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Search for an existing report before starting a new thread. Check the regressions mailing-list archives and regzbot’s interface so an existing report does not get split across duplicate discussions.
Reproduce and narrow it down
Describe what happens in practical terms: what you tried, what you expected, what occurred instead, and the relevant hardware, software, and kernel setup. If possible, reproduce the failure using comparable configurations and check whether it also occurs on a current kernel build.
When you can build and test kernels, bisection may narrow the problem to a particular change. The kernel’s bug verification and bisection guide describes how to set up kernels for investigation. Include the culprit commit ID in the report if you identify one.
How to send a useful report
- Use a clear subject. Start the subject with
[REGRESSION]and briefly name the affected behavior. - Copy the regressions list. Send the report to [email protected]. Include the last known-good and first affected kernel versions, the practical impact, and the setup needed to understand or reproduce the problem.
- Include the right maintainers and lists. If the issue first appears in a bug tracker, forward it to the regressions list while copying the relevant maintainer and subsystem mailing list. For a regression within a stable or longterm kernel series, also copy [email protected]. If bisection found a culprit, include the recipients named in its commit message’s
Signed-off-by:lines. - Ask regzbot to track it. Add a standalone line to the report such as
#regzbot introduced: vX..vY, replacing the example range with the last known-good and first affected versions. If you know the culprit commit, use it to identify the introduction instead. This optional, recommended command helps make the report visible in tracking and weekly reports.
For the exact current reporting instructions and regzbot syntax, consult the official reporting regressions page.
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How maintainers should keep a report on track
If a regression report reaches a developer without the regressions list copied, the developer should bring the list into the conversation. When the report originates in a bug tracker, forward it to the list. A developer can ask regzbot to track the parent report by replying with a command such as #regzbot ^introduced: ...; the caret tells the bot to treat the parent report as the issue being tracked. See the kernel’s regression handling guidance.
When submitting a fix, connect it to the report with a Closes: tag and identify the offending commit with a Fixes: tag. If the culprit was merged in an earlier development cycle and the fix should be backported, add Cc: [email protected]. These tags help maintainers, distributions, and tracking tools connect the report, cause, and resolution.
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How quickly should a regression be fixed?
The kernel user guide says most regressions should be fixed within two weeks. Developer guidance says affected users ideally should not have to remain on an outdated, potentially insecure kernel for more than three weeks after the culprit is identified, and preferably for less than two. These are expectations, not guaranteed deadlines; the appropriate urgency depends on impact, how long the problem has existed, and the kernel series. Severe or widespread regressions call for faster action. The guidance is documented in the pages on reporting regressions and handling regressions.
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