LWN on Kernel (Paywall Lapsed): Linux 5.6, better tools for kernel developers, and kernel operations structures in BPF
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The rest of the 5.6 merge window
Linus Torvalds released the 5.6-rc1 prepatch and closed the merge window on February 9; at that point, 10,780 non-merge changesets had been pulled into the mainline repository for 5.6. That is substantially less than recent development cycles (14,350 for 5.5, 14,619 for 5.4), but is similar to what was going on at this time last year (10,843 for 5.0-rc1 in January 2019). About 6,000 of those changes were pulled since the first 5.6 merge-window article was written; read on for what was included in those changes.
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Better tools for kernel developers
By many accounts, the kernel project uses outdated tooling, far behind the state of the art that Kids Today tend to favor. The kernel's workflow has worked well (enough) for years, but there are signs that it may not be sustainable indefinitely. As a result, there has been an ongoing conversation about improving the kernel's workflow, but little has changed so far. The posting of a simple tool called get-lore-mbox is a sign that the rate of change may be about to increase.
The kernel project's reliance on email strikes many as quaint and antiquated. It may indeed partly be a natural outcome of the aging nature of the kernel community; many of the developers there, especially in the important maintainer positions, got started well before tools like web-based Git forges existed. Indeed, some of them got started using punch cards and may still be unconvinced of the virtues of, say, text editors. But the truth of the matter is that there are a number of good reasons for the kernel community's continued reliance on email; there is little else that can handle a community of that size and diversity.
So, while it seems that the future of email (as opposed to, say, proprietary services like Gmail) is uncertain at best, the path toward a replacement in the kernel community is unclear. Developers will have to be convinced that any new tools will make their lives better, not worse; busy maintainers have little patience for "improvements" that slow things down.
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Kernel operations structures in BPF
One of the more eyebrow-raising features to go into the 5.6 kernel is the ability to load TCP congestion-control algorithms as BPF programs; networking developer Toke Høiland-Jørgensen described it as a continuation of the kernel's "march towards becoming BPF runtime-powered microkernel". On its face, congestion control is a significant new functionality to hand over to BPF, taking it far beyond its existing capabilities. When one looks closer, though, one's eyebrow altitude may well increase further; the implementation of this feature breaks new ground in a couple of areas.
The use case for this feature seems clear enough. There are a number of such algorithms in use, each of which is suited for a different networking environment. There may be good reasons to distribute an updated or improved version of an algorithm and for recipients to be able to make use of it without building a new kernel or even rebooting. Networking developers can certainly benefit from being able to play with congestion-control code on the fly. One could argue that congestion control is not conceptually different from other tasks, such as flow dissection or IR protocol decoding, that can be done with BPF now — but congestion control does involve a rather higher level of complexity.A look at the patch set posted by Martin KaFai Lau reveals that what has been merged for 5.6 is not just a mechanism for hooking in TCP congestion-control algorithms; it is far more general than that. To be specific, this new infrastructure can be used to allow a BPF program to replace any "operations structure" — a structure full of function pointers — in the kernel. It is, at this point, only capable of replacing the tcp_congestion_ops structure used for congestion control; experience suggests, though, that other uses will show up sooner rather than later.
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