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diff --git a/Documentation/admin-guide/reporting-regressions.rst b/Documentation/admin-guide/reporting-regressions.rst new file mode 100644 index 000000000000..6fbd24ceb3bf --- /dev/null +++ b/Documentation/admin-guide/reporting-regressions.rst @@ -0,0 +1,439 @@ +.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0) +.. [see the bottom of this file for redistribution information] + +Reporting regressions ++++++++++++++++++++++ + +"*We don't cause regressions*" is the first rule of Linux kernel development; +Linux founder and lead developer Linus Torvalds established it himself and +ensures it's obeyed. + +This document describes what the rule means for users and how the Linux kernel's +development model ensures to address all reported regressions; aspects relevant +for kernel developers are left to Documentation/process/handling-regressions.rst. + + +The important bits (aka "TL;DR") +================================ + +#. It's a regression if something running fine with one Linux kernel works worse + or not at all with a newer version. Note, the newer kernel has to be compiled + using a similar configuration; the detailed explanations below describes this + and other fine print in more detail. + +#. Report your issue as outlined in Documentation/admin-guide/reporting-issues.rst, + it already covers all aspects important for regressions and repeated + below for convenience. Two of them are important: start your report's subject + with "[REGRESSION]" and CC or forward it to `the regression mailing list + <https://lore.kernel.org/regressions/>`_ (regressions@lists.linux.dev). + +#. Optional, but recommended: when sending or forwarding your report, make the + Linux kernel regression tracking bot "regzbot" track the issue by specifying + when the regression started like this:: + + #regzbot introduced v5.13..v5.14-rc1 + + +All the details on Linux kernel regressions relevant for users +============================================================== + + +The important basics +-------------------- + + +What is a "regression" and what is the "no regressions rule"? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +It's a regression if some application or practical use case running fine with +one Linux kernel works worse or not at all with a newer version compiled using a +similar configuration. The "no regressions rule" forbids this to take place; if +it happens by accident, developers that caused it are expected to quickly fix +the issue. + +It thus is a regression when a WiFi driver from Linux 5.13 works fine, but with +5.14 doesn't work at all, works significantly slower, or misbehaves somehow. +It's also a regression if a perfectly working application suddenly shows erratic +behavior with a newer kernel version; such issues can be caused by changes in +procfs, sysfs, or one of the many other interfaces Linux provides to userland +software. But keep in mind, as mentioned earlier: 5.14 in this example needs to +be built from a configuration similar to the one from 5.13. This can be achieved +using ``make olddefconfig``, as explained in more detail below. + +Note the "practical use case" in the first sentence of this section: developers +despite the "no regressions" rule are free to change any aspect of the kernel +and even APIs or ABIs to userland, as long as no existing application or use +case breaks. + +Also be aware the "no regressions" rule covers only interfaces the kernel +provides to the userland. It thus does not apply to kernel-internal interfaces +like the module API, which some externally developed drivers use to hook into +the kernel. + +How do I report a regression? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Just report the issue as outlined in +Documentation/admin-guide/reporting-issues.rst, it already describes the +important points. The following aspects outlined there are especially relevant +for regressions: + + * When checking for existing reports to join, also search the `archives of the + Linux regressions mailing list <https://lore.kernel.org/regressions/>`_ and + `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_. + + * Start your report's subject with "[REGRESSION]". + + * In your report, clearly mention the last kernel version that worked fine and + the first broken one. Ideally try to find the exact change causing the + regression using a bisection, as explained below in more detail. + + * Remember to let the Linux regressions mailing list + (regressions@lists.linux.dev) know about your report: + + * If you report the regression by mail, CC the regressions list. + + * If you report your regression to some bug tracker, forward the submitted + report by mail to the regressions list while CCing the maintainer and the + mailing list for the subsystem in question. + + If it's a regression within a stable or longterm series (e.g. + v5.15.3..v5.15.5), remember to CC the `Linux stable mailing list + <https://lore.kernel.org/stable/>`_ (stable@vger.kernel.org). + + In case you performed a successful bisection, add everyone to the CC the + culprit's commit message mentions in lines starting with "Signed-off-by:". + +When CCing for forwarding your report to the list, consider directly telling the +aforementioned Linux kernel regression tracking bot about your report. To do +that, include a paragraph like this in your mail:: + + #regzbot introduced: v5.13..v5.14-rc1 + +Regzbot will then consider your mail a report for a regression introduced in the +specified version range. In above case Linux v5.13 still worked fine and Linux +v5.14-rc1 was the first version where you encountered the issue. If you +performed a bisection to find the commit that caused the regression, specify the +culprit's commit-id instead:: + + #regzbot introduced: 1f2e3d4c5d + +Placing such a "regzbot command" is in your interest, as it will ensure the +report won't fall through the cracks unnoticed. If you omit this, the Linux +kernel's regressions tracker will take care of telling regzbot about your +regression, as long as you send a copy to the regressions mailing lists. But the +regression tracker is just one human which sometimes has to rest or occasionally +might even enjoy some time away from computers (as crazy as that might sound). +Relying on this person thus will result in an unnecessary delay before the +regressions becomes mentioned `on the list of tracked and unresolved Linux +kernel regressions <https://linux-regtracking.leemhuis.info/regzbot/>`_ and the +weekly regression reports sent by regzbot. Such delays can result in Linus +Torvalds being unaware of important regressions when deciding between "continue +development or call this finished and release the final?". + +Are really all regressions fixed? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Nearly all of them are, as long as the change causing the regression (the +"culprit commit") is reliably identified. Some regressions can be fixed without +this, but often it's required. + +Who needs to find the root cause of a regression? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Developers of the affected code area should try to locate the culprit on their +own. But for them that's often impossible to do with reasonable effort, as quite +a lot of issues only occur in a particular environment outside the developer's +reach -- for example, a specific hardware platform, firmware, Linux distro, +system's configuration, or application. That's why in the end it's often up to +the reporter to locate the culprit commit; sometimes users might even need to +run additional tests afterwards to pinpoint the exact root cause. Developers +should offer advice and reasonably help where they can, to make this process +relatively easy and achievable for typical users. + +How can I find the culprit? +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Perform a bisection, as roughly outlined in +Documentation/admin-guide/reporting-issues.rst and described in more detail by +Documentation/admin-guide/bug-bisect.rst. It might sound like a lot of work, but +in many cases finds the culprit relatively quickly. If it's hard or +time-consuming to reliably reproduce the issue, consider teaming up with other +affected users to narrow down the search range together. + +Who can I ask for advice when it comes to regressions? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Send a mail to the regressions mailing list (regressions@lists.linux.dev) while +CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the +issue might better be dealt with in private, feel free to omit the list. + + +Additional details about regressions +------------------------------------ + + +What is the goal of the "no regressions rule"? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Users should feel safe when updating kernel versions and not have to worry +something might break. This is in the interest of the kernel developers to make +updating attractive: they don't want users to stay on stable or longterm Linux +series that are either abandoned or more than one and a half years old. That's +in everybody's interest, as `those series might have known bugs, security +issues, or other problematic aspects already fixed in later versions +<http://www.kroah.com/log/blog/2018/08/24/what-stable-kernel-should-i-use/>`_. +Additionally, the kernel developers want to make it simple and appealing for +users to test the latest pre-release or regular release. That's also in +everybody's interest, as it's a lot easier to track down and fix problems, if +they are reported shortly after being introduced. + +Is the "no regressions" rule really adhered in practice? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +It's taken really seriously, as can be seen by many mailing list posts from +Linux creator and lead developer Linus Torvalds, some of which are quoted in +Documentation/process/handling-regressions.rst. + +Exceptions to this rule are extremely rare; in the past developers almost always +turned out to be wrong when they assumed a particular situation was warranting +an exception. + +Who ensures the "no regressions" is actually followed? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The subsystem maintainers should take care of that, which are watched and +supported by the tree maintainers -- e.g. Linus Torvalds for mainline and +Greg Kroah-Hartman et al. for various stable/longterm series. + +All of them are helped by people trying to ensure no regression report falls +through the cracks. One of them is Thorsten Leemhuis, who's currently acting as +the Linux kernel's "regressions tracker"; to facilitate this work he relies on +regzbot, the Linux kernel regression tracking bot. That's why you want to bring +your report on the radar of these people by CCing or forwarding each report to +the regressions mailing list, ideally with a "regzbot command" in your mail to +get it tracked immediately. + +Is it a regression, if the issue can be avoided by updating some software? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Almost always: yes. If a developer tells you otherwise, ask the regression +tracker for advice as outlined above. + +Is it a regression, if a newer kernel works slower or consumes more energy? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Yes, but the difference has to be significant. A five percent slow-down in a +micro-benchmark thus is unlikely to qualify as regression, unless it also +influences the results of a broad benchmark by more than one percent. If in +doubt, ask for advice. + +Is it a regression, if an external kernel module breaks when updating Linux? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +No, as the "no regression" rule is about interfaces and services the Linux +kernel provides to the userland. It thus does not cover building or running +externally developed kernel modules, as they run in kernel-space and hook into +the kernel using internal interfaces occasionally changed. + +How are regressions handled that are caused by security fixes? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +In extremely rare situations security issues can't be fixed without causing +regressions; those fixes are given way, as they are the lesser evil in the end. +Luckily this middling almost always can be avoided, as key developers for the +affected area and often Linus Torvalds himself try very hard to fix security +issues without causing regressions. + +If you nevertheless face such a case, check the mailing list archives if people +tried their best to avoid the regression. If not, report it; if in doubt, ask +for advice as outlined above. + +What happens if fixing a regression is impossible without causing another? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Sadly these things happen, but luckily not very often; if they occur, expert +developers of the affected code area should look into the issue to find a fix +that avoids regressions or at least their impact. If you run into such a +situation, do what was outlined already for regressions caused by security +fixes: check earlier discussions if people already tried their best and ask for +advice if in doubt. + +A quick note while at it: these situations could be avoided, if people would +regularly give mainline pre-releases (say v5.15-rc1 or -rc3) from each +development cycle a test run. This is best explained by imagining a change +integrated between Linux v5.14 and v5.15-rc1 which causes a regression, but at +the same time is a hard requirement for some other improvement applied for +5.15-rc1. All these changes often can simply be reverted and the regression thus +solved, if someone finds and reports it before 5.15 is released. A few days or +weeks later this solution can become impossible, as some software might have +started to rely on aspects introduced by one of the follow-up changes: reverting +all changes would then cause a regression for users of said software and thus is +out of the question. + +Is it a regression, if some feature I relied on was removed months ago? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +It is, but often it's hard to fix such regressions due to the aspects outlined +in the previous section. It hence needs to be dealt with on a case-by-case +basis. This is another reason why it's in everybody's interest to regularly test +mainline pre-releases. + +Does the "no regression" rule apply if I seem to be the only affected person? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +It does, but only for practical usage: the Linux developers want to be free to +remove support for hardware only to be found in attics and museums anymore. + +Note, sometimes regressions can't be avoided to make progress -- and the latter +is needed to prevent Linux from stagnation. Hence, if only very few users seem +to be affected by a regression, it for the greater good might be in their and +everyone else's interest to lettings things pass. Especially if there is an +easy way to circumvent the regression somehow, for example by updating some +software or using a kernel parameter created just for this purpose. + +Does the regression rule apply for code in the staging tree as well? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Not according to the `help text for the configuration option covering all +staging code <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/staging/Kconfig>`_, +which since its early days states:: + + Please note that these drivers are under heavy development, may or + may not work, and may contain userspace interfaces that most likely + will be changed in the near future. + +The staging developers nevertheless often adhere to the "no regressions" rule, +but sometimes bend it to make progress. That's for example why some users had to +deal with (often negligible) regressions when a WiFi driver from the staging +tree was replaced by a totally different one written from scratch. + +Why do later versions have to be "compiled with a similar configuration"? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Because the Linux kernel developers sometimes integrate changes known to cause +regressions, but make them optional and disable them in the kernel's default +configuration. This trick allows progress, as the "no regressions" rule +otherwise would lead to stagnation. + +Consider for example a new security feature blocking access to some kernel +interfaces often abused by malware, which at the same time are required to run a +few rarely used applications. The outlined approach makes both camps happy: +people using these applications can leave the new security feature off, while +everyone else can enable it without running into trouble. + +How to create a configuration similar to the one of an older kernel? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Start your machine with a known-good kernel and configure the newer Linux +version with ``make olddefconfig``. This makes the kernel's build scripts pick +up the configuration file (the ".config" file) from the running kernel as base +for the new one you are about to compile; afterwards they set all new +configuration options to their default value, which should disable new features +that might cause regressions. + +Can I report a regression I found with pre-compiled vanilla kernels? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +You need to ensure the newer kernel was compiled with a similar configuration +file as the older one (see above), as those that built them might have enabled +some known-to-be incompatible feature for the newer kernel. If in doubt, report +the matter to the kernel's provider and ask for advice. + + +More about regression tracking with "regzbot" +--------------------------------------------- + +What is regression tracking and why should I care about it? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Rules like "no regressions" need someone to ensure they are followed, otherwise +they are broken either accidentally or on purpose. History has shown this to be +true for Linux kernel development as well. That's why Thorsten Leemhuis, the +Linux Kernel's regression tracker, and some people try to ensure all regression +are fixed by keeping an eye on them until they are resolved. Neither of them are +paid for this, that's why the work is done on a best effort basis. + +Why and how are Linux kernel regressions tracked using a bot? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Tracking regressions completely manually has proven to be quite hard due to the +distributed and loosely structured nature of Linux kernel development process. +That's why the Linux kernel's regression tracker developed regzbot to facilitate +the work, with the long term goal to automate regression tracking as much as +possible for everyone involved. + +Regzbot works by watching for replies to reports of tracked regressions. +Additionally, it's looking out for posted or committed patches referencing such +reports with "Link:" tags; replies to such patch postings are tracked as well. +Combined this data provides good insights into the current state of the fixing +process. + +How to see which regressions regzbot tracks currently? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Check out `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_. + +What kind of issues are supposed to be tracked by regzbot? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The bot is meant to track regressions, hence please don't involve regzbot for +regular issues. But it's okay for the Linux kernel's regression tracker if you +involve regzbot to track severe issues, like reports about hangs, corrupted +data, or internal errors (Panic, Oops, BUG(), warning, ...). + +How to change aspects of a tracked regression? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +By using a 'regzbot command' in a direct or indirect reply to the mail with the +report. The easiest way to do that: find the report in your "Sent" folder or the +mailing list archive and reply to it using your mailer's "Reply-all" function. +In that mail, use one of the following commands in a stand-alone paragraph (IOW: +use blank lines to separate one or multiple of these commands from the rest of +the mail's text). + + * Update when the regression started to happen, for example after performing a + bisection:: + + #regzbot introduced: 1f2e3d4c5d + + * Set or update the title:: + + #regzbot title: foo + + * Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of + the issue or a fix are discussed::: + + #regzbot monitor: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/ + #regzbot monitor: https://bugzilla.kernel.org/show_bug.cgi?id=123456789 + + * Point to a place with further details of interest, like a mailing list post + or a ticket in a bug tracker that are slightly related, but about a different + topic:: + + #regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789 + + * Mark a regression as invalid:: + + #regzbot invalid: wasn't a regression, problem has always existed + +Regzbot supports a few other commands primarily used by developers or people +tracking regressions. They and more details about the aforementioned regzbot +commands can be found in the `getting started guide +<https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_ and +the `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_ +for regzbot. + +.. + end-of-content +.. + This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top + of the file. If you want to distribute this text under CC-BY-4.0 only, + please use "The Linux kernel developers" for author attribution and link + this as source: + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/reporting-regressions.rst +.. + Note: Only the content of this RST file as found in the Linux kernel sources + is available under CC-BY-4.0, as versions of this text that were processed + (for example by the kernel's build system) might contain content taken from + files which use a more restrictive license. |