As mentioned in the previous discussion on bisection, Fedora maintains a series of patches on top of the kernel.org kernel (Kernel jargon: if you here something referred to as "vanilla kernel" it's referring to a kernel from kernel.org without any additional patches). The patch count for rawhide as of this post was ~50. In a previous job, the trees I worked on had ~20,000 patches and over 1 million lines of code difference from vanilla. I was significantly less hands on with the maintenance of that 1 million line tree but I still observed how it did and didn't scale. These certainly represent some of the extremes of trees but ultimately the tasks need to be done to maintain those tree even if how they happen can vary.
Adding a patch that was written for a tree specifically is one of the easier tasks. For Fedora, one of the maintainers copies the patch into the pkg-git directory, updates the spec file, and pushes the git branch. This works for Fedora because we have a relatively low rate of patches coming in directly. As the number of contributors to a tree grows, having individual maintainers do this work becomes harder and harder. Maintainers now have to do this work multiple times and make sure the individual patches that are being merged don't conflict with each other. One solution can be to switch to a continuous integration system coupled with a review system like gerrit. Of course, then you need people to do the work of maintaining that CI system. (Running a CI system can be a thankless job. Thank you to all who do this work. Have you thanked your DevOps today?)
Inevitably, it will be necessary to bring in a patch or feature that wasn't written against your branch. Backport time! Sometimes the feature will apply cleanly to the branch without any conflicts. Commonly though, patches will not and then the maintainer needs to figure out what to do. Conflict resolution requires thought about the code. The thoughts that are going through my head when I'm trying to do a backport include:
- Which part of the code has the conflict? - What's actually changed to introduce the conflict? - Is this a feature addition or a bug fix? - Am I dropping parts of the code? If so, what am I dropping? - Is this changing code flow? structures? #defines? - Can I take both parts? Just one? Which one? - Will this change affect any other parts of the code base? - What is the patch I'm trying to apply actually doing?
That last point is the most important. Individuals who are working on backports need to have some degree of familiarity with the the code base. Without that familiarity, it's difficult to ensure that the applied patch is still doing the same thing. Even with familiarity it's still easy to get wrong (I once did a backport improperly and introduced a page accounting bug that went unnoticed for months). The bigger the difference between the patch base and tree, the more work is required to bring it in. Backports are necessary but are not cheap or risk free.
Eventually the time may come to bring a branch to a new kernel version. The ultimate goal is to have a tree on the kernel you want with the features you want with the git history you want. This leaves a lot of options open as to how you produce a tree which meets those requirements. If you're lucky, some of the patches in the tree will already be present in the new kernel which means they no longer need to be maintained. Inevitably, some of the patches are not yet upstream which means that conflicts for each patch/file must be worked through via the backport logic. Once all conflicts are resolved, new features in the kernel need to be evaluated. A Fedora major rebase can take about a day or two depending on conflicts and the type. Rawhide typically takes an hour or two during the merge window, less during -rc times. A tree with lots of out of tree patches and drivers can take weeks to get compiling and then more weeks to test and verify.
What's the conclusion here? Maintaining kernel trees and branches takes developer effort and time. This work can easily fill a full time job. Getting code into the mainline reduces some of the maintenance burden for upgrading to a new kernel version. For some trees though there may always be patches which need to be carried. Ultimately, branch work is a trade off of changing the code base for features/fixes vs. not changing and spending that development time elsewhere. For companies that have their own trees, throw in some buzzwords when having this discussion ("business need", "value add", "deliverables", "customer ask", "resources"). The two sides are constantly at odds and it only gets worse the older the branch gets. Human problems are the hardest to solve in technology.