regzbot: 'the design' aka 'how is this thing supposed to work'

The bot this project will build got a name: "regzbot". That makes it a good time to explain how it will look like and how it's supposed to establish proper regression tracking in the Linux kernel development process. So here is a simple example that is more concrete than the vague introduction post. Below text was also posted to Linus, Greg and a few mailing list as RFC.

The ideal case is simple. Someone reports a regression to the mailing list recently created for regression reports on my behalf. There the user includes a tag like this:

1#regzb introduced: 94a632d91ad1 ("usc: xhbi-foo: check bar_params earlier")

That will make regzbot add the report to its list of regressions it tracks, which among other will make it store the mail's message-id (let's assume it's xt6uzpqtaqru6pmh@earth.solsystem). Ideally some developer within a few days will fix the regression with a patch. When doing so, they already often include a tag linking to the report:


Regzbot will notice this tag refers to the regression it tracks and automatically close the entry.

That's it already. The regression was tracked with:

  • minimal overhead for the reporter
  • no additional overhead for the developers – only something they ought to do already became more important

Invisible ideally

In the ideal case regzbot thus seems to be of no use. But obviously things will be anything else than ideal quite often – for example when nobody fixes the reported regression.

The webpages that Regzbot will generate (see below for a mockup) will show this. They among others are meant for release managers like Linus or Greg to check how things stand. That makes it easy for them to simply revert the culprit change to resolve the regression; in other situations they might decide to delay a release to get crucial regressions solved.

And that's what regression tracking is about: providing a view into the state of things with regards to regressions, as that's the important thing missing in Linux kernel development right now.

That can't be all

Of course the world is more complicated than the simple example scenario above, as the devil is always in the details. The three most obvious problems the initial ideal scenario left aside:

  • The reporter doesn't specify the #regzb tag at all. Regzbot can't do anything about it, it sadly won't have visionary power and a AI engine any time soon. Some human (for a while that often will be me) thus needs to reply with the tag with a proper reply-to to the report to make regboz track it.

  • The commit causing the regression is unknown to the reporter. In that case the tag should mention the span when the regression was introduced:

1#regzb introduced: v5.7..v5.8-rc1
  • The developer who fixes the issue forgets to place the "Link:" tag, which can't be added once committed. In that case some human needs to reply to the thread with the initial report with a tag like this:
1#regzb Fixed-by: c39667ddcfd5 

How will it look

Here is a mockup:


You'll notice a few things:

  • regressions for mainline kernel will be shown on a different page than those in stable and longterm kernels, as those series are handled by different release maintainers.

  • regressions where the culprit is known get the top spot, as changes causing one can sometimes simply be reverted withing seconds to fix the regression.

  • the second spot is for regressions in the current cycle, as contrary to those in previous release there is still time to fix those before the next release.

  • Regzbot will try to monitor the process between reporting and fixing and provide links to lookup details. Regzbot will thus watch the thread where the regression was reported and show when it noticed the last activity; it will also look out for #regszb Link: and Link: tags in patch submissions and linux-next. That way release managers can immediately see if things stalled after the regression was reported; it also allows them to see if developers are working on a fix and how far it got in the machinery. If the causing commit is known, the webview obviously will link to it as well.

  • regressions where nothing happened for a while will be moved to the "dormant" page, to prevent the status page from getting filled by reports that obviously nobody cares about anymore. Reporters will be told about this by mail to give them a chance to provide a fresh status update to get things rolling again.

Even more problems in the details

Regzbot on purpose will lack many features found in traditional bug trackers: it's meant to be a simple tool acting in the background without much overhead, as it doesn't want to become yet another bug tracker. Nevertheless, it will need a few features they typically offer. Those will be usable via tags that need to be dropped into mails send in direct or indirect reply to the mail with the report:

  • Mark a report as a duplicate of another or revert such a marking:
1#regzb dup:
2#regzb undup
  • Mark a report as invalid.
1#regzb invalid: Turned out it never worked
  • generate a new title
1#regzb new-title: Insert better description of the regression
  • the initially mentioned tag can be used to also change the range or the commit that is causing the regression:
1#regzb introduced: v5.7..v5.8-rc1
  • Tell regzbot that a discussion is related to a tracked regression:
1#regzb Link:

In the long run this is supposed to work in both directions, so you can use it in a thread started by a regression report to link to some other discussion or vice versa.

Implications and hidden aspects

There are a few things of note:

  • The plan for now is to not have a tag like #regzb unfix: in case it turns out a commit did not fix a regression it's likely better to start with a fresh report anyway. That forces someone to explain the current state of things including the history clearly and straight forward; that makes things a lot easier to follow for others in these situations and thus is a good thing.

  • regzbot works without a public unique-id, as it uses the URL of the report instead and keeps any eye on is using the mail's message-id (say 20210406135151.xt6uzpqtaqru6pmh@earth.solsystem).

  • regzbot won't be able to handle regressions reported to a mailing list thread that is already tracked by regzbot, as it will assume all mails in a thread are related to the earlier report. In that case the reporter must be asked to start a new mailing list thread for the second regression. But that's quite normal, as a similar approach is needed when somebody reports an issue deep in a bug tracker ticket that was crated for a totally different issue.

  • Initially it won't be possible to track reports that are filed in bug trackers; but this use-case will be kept in mind during the design to make sure such a functionality can be added later easily.

  • developer when fixing a regression with a bisected "#regzb introduced:" tag can simply do s/#regzb introduced:/Fixes:/ to get a tag they are supposed to add.

  • regression in stable and longterm kernels sometimes affect multiple versions, for example if a patch that works fine in mainline was backported to the longterm kernel 5.10 and 5.4 – but causes problems in both, as something required by the patch is missing in those lines. How this will be solved exactly remains to be seen, maybe like this:

1#regzb Introduced: c39667ddcfd6 e39667ddcfd1 ("usc: xhbi-foo: check bar_params a little later again")

Then regzbot can look those commits up and from that determine the affected versions. Obviously the reporter will likely not be aware of it, hence it's likely that the stable maintainer or the developer need to send a mail to make regzbot aware that this regression affects multiple versions.

  • Regzbot will need to be able to work with mails where mailers placed a linebreak into the text that follows the #regzb tag. This will be tricky, but is doable.

  • to keep things simple there are neither authentication nor restrictions for now, so anyone could mess things up by sending mails to an open list and using those tags. If that against expectations turns out to become a problem some restrictions will need to be put in place, for example to allow changes only from email addresses that (1) are on an allow list, (2) participated in the discussion or (3) have commits in the kernel. People could still forge complete mails including "From", but that's quite some work for not much to gain (except for messing regression tracking up).


The rough initial idea had been to reuse parts of the syzbot golang source code, which already has an email interface similar to the one regzbot needs. But the closer I looked, the more I came to the conclusion that writing something in python is easier and better (even if that means I need to bring my really rusty python skills up to speed). That also has the benefit that python afaics is preferred by the admins, which would make it more attractive for them to host the bot later.

The focus will be to properly establishing regression tracking with regszbot first. All features not strictly needed will thus be left out first to focus on what's most important. I'll also provide documentation. And to see how things go I ill use the bot myself to track regressions as I did a few years ago. But just like any other tracking solution it will always need some hand-holding...

[Update 2021-04-29]

Posting a variant of above text to LKML let to a small discussion where a few important points were raised – and best written down here to make sure they are not forgotten:

  • GregKH seemed to like the concept. He indicated the aspect "one badly backported patch might cause regression in various stable and longterm kernels" is not something regzbot should worry about too much initially: the stable maintainers already have something to look the details up when something like that happens.

  • Takashi wondered how to populate a report from distro bug to the list of tracked issues. For now forwarding to the regressions mailing list with the appropriate tag must suffice, but it's a good point and makes above-mentioned aspect reg. bug tracker more important. So that feature likely should be added rather sooner than later.

  • Guillaume mentioned that KernelCI and KCIDB have regression tracking on their roadmap as well, which will likely create some overlap with regzbot and thus lead to duplicates (a problem which Mark had kinda raised earlier already). To make sure that doesn't become a problem the current plan is for regzbot to stay away from reports sent by CI systems, unless some developer wants to see them tracked by regzbot. That also has the benefit that all the tracked issues are problems that some human faces or thinks are crucial to track, and not some problem a CI system turned up that might or might not lead to practical problems. That's the long story short, see the two last links and the replies to it for details.


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