Linux Kernel Regression: What to Verify Before Filing

Before you file a Linux kernel regression report, you must verify that the problem is truly caused by a kernel change, not by hardware, userspace, or a configuration issue. This article explains the essential checks and information to gather, so your report is useful to developers and increases the chance of a fix.

Formula

The calculator above is a placeholder; this is a tech how-to article, so no formula applies. The real content focuses on kernel regression verification steps.

Background: What Is a Kernel Regression?

A kernel regression is a bug introduced by a change in the Linux kernel that causes a previously working feature to break. This could be a driver failure, a performance drop, a boot problem, or a system crash. Regressions are taken seriously by kernel developers, but they need clear evidence to act. Filing a vague report with no reproduction steps or bisection results often leads to the issue being ignored or closed.

Step 1: Confirm the Kernel Version

You need to know the exact kernel version where the problem started. Run uname -r to get the current version. Also note the distribution and any patches applied. If you are using a distribution kernel, the regression might be caused by a distro patch, not an upstream change. Check the distribution’s changelog or bug tracker for known issues.

Step 2: Test the Previous Kernel

The most direct verification is to boot an older kernel that you know worked. Most distributions keep at least one previous kernel in the bootloader menu. If the problem disappears with the older kernel, it is likely a regression. If it persists, the issue may be hardware or configuration related. If you cannot boot an older kernel, you can use a live USB with a different kernel version to test.

Step 3: Check Hardware and Firmware

Before blaming the kernel, rule out hardware failure. Run memory tests (like memtest86+), check disk health with smartctl, and verify that the hardware is properly seated. Firmware updates (BIOS/UEFI, SSD firmware, GPU firmware) can also cause issues that look like kernel regressions. A recent firmware update might have changed behavior that conflicts with the kernel.

Step 4: Examine Kernel Logs

Kernel logs often contain error messages that point to the cause. Use dmesg or journalctl -k to see messages around the time of the problem. Look for warnings, errors, oops, or tracebacks. Pay attention to messages related to the hardware or driver that is failing. Include these logs in your report, but trim them to the relevant part.

Step 5: Test with a Minimal Configuration

Sometimes the regression only appears with a specific configuration or when certain modules are loaded. Try booting with kernel parameters like nomodeset (for graphics issues) or acpi=off (for ACPI-related problems). If the issue disappears, it narrows down the culprit. Also, try disabling unnecessary hardware in the BIOS to see if a specific device is triggering the problem.

Step 6: Perform a Bisection

If you can compile kernels, a bisection is the gold standard for locating the introducing commit. You can use git bisect on the kernel source tree. Start with a known good and a known bad commit, then test each midpoint. This requires time and effort, but it dramatically speeds up the fix. If you cannot bisect, at least provide the first kernel version where the problem appears.

Step 7: Gather System Information

When filing the report, include: kernel version, distribution, CPU model, motherboard, GPU, network card, and any other relevant hardware. Also mention the exact steps to reproduce the issue, and whether it happens on a fresh boot or after a specific action. The more detail, the better.

Common Mistakes to Avoid

• Filing a report without testing an older kernel.
• Ignoring hardware faults that mimic kernel bugs.
• Not providing logs or error messages.
• Using a distribution kernel without checking if the distro has patched it.
• Assuming a regression is upstream when it might be a config issue.

Where to File

For upstream kernel issues, use the bugzilla.kernel.org or the relevant mailing list (e.g., LKML). For distribution kernels, use the distribution’s bug tracker. Include the phrase “regression” in the title, and mention the last known good version. If you are not sure, ask on a forum or IRC channel first.

Frequently Asked Questions

Q: How do I know if a problem is a kernel regression or a hardware issue?

Test with a different kernel version. If the problem disappears with an older kernel, it is likely a regression. If it persists, it could be hardware or firmware.

Q: What if I can’t boot an older kernel?

Use a live USB with a different kernel version. Most live distributions allow you to test without installing. If you still see the problem, it might not be a kernel regression.

Q: Should I always bisect?

Bisecting is not required, but it is very helpful. If you can’t, provide the first kernel version where the issue appears. That alone helps developers narrow down the change.

Q: My distribution kernel is older than the upstream. Is that a problem?

Yes, distribution kernels often have patches and backports. Check if the issue is already fixed in a newer upstream kernel. If it is, you might just need to wait for a distro update.

Q: What logs should I include in the report?

Include the relevant part of dmesg or journalctl -k that shows errors or warnings. Also include the output of lspci -v and lsusb to identify hardware.

Q: Can a regression be caused by a userspace change?

Yes, sometimes a userspace update (like a new version of systemd or a driver in userspace) can break things. Test with a different userspace (e.g., a different distribution) to confirm the kernel is at fault.

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