The 5.12 kernel has been released

flussence — Wed, 26 May 2021 12:02:08 +0000

Some closure: I've now successfully convinced an LTO kernel to build and boot by turning the modules mentioned in the compilation errors into built-ins. The bzImage + /lib/modules/ is a few KB smaller in total than without LTO, but it seems like noise.

Overall, I feel like there's much bigger kernel size wins to be made simply by breaking up monolithic GPU drivers into more granular model ranges...

The 5.12 kernel has been released

flussence — Wed, 28 Apr 2021 12:57:26 +0000

I've been playing around with the LTO stuff, unsuccessfully. Here's what I've found:

It's next to impossible to discover unless you're already aware of the exact git commit message with the magic incantation. I couldn't figure out what was locking out the settings in nconfig, even with the symbol search's usually useful metadata (it's empty for HAVE_LTO_CLANG), and had to resort to grepping makefiles. To save others the time, the answer's `make LLVM=1 LLVM_IAS=1` — both vars are needed on every config and compile command.

Having TRIM_UNUSED_KSYMS enabled caused the build to explode with undefined module symbols in modpost, though it looks like the kernel image itself built successfully. The symbols are all internal-sounding ones: __x86_indirect_thunk_r11, __SCT__preempt_schedule{,_notrace}, and __{get,put}_user_{1,4,8}.

The help text for that option explicitly suggests this is a thing that's supposed to work with LTO. It doesn't seem to be caused solely by that option, as I can't reproduce the error from a fresh allnoconfig just by flipping it on. I have no idea how to efficiently narrow down the source and I gave up after hours of trying the hard way. Having it disabled succeeds, but the resulting kernel is massive.

A plain kernel built with clang and no LTO has no problems though, I've got two machines running on it now.

The 5.12 kernel has been released

flussence — Mon, 26 Apr 2021 11:15:34 +0000

Okay, preemption model switching at *runtime*? That's a really cool surprise. (I know you're not meant to do it at runtime, but still)

LWN: Comments on "The 5.12 kernel has been released"

The 5.12 kernel has been released

The 5.12 kernel has been released

The 5.12 kernel has been released