thinLTO is linking with one thread?

[nickdesaulniers]

Doing an x86 defconfig build with CONFIG_LTO_CLANG_THIN=y enabled, I'm seeing only 1 thread being used by LLD during LD vmlinux.o. Is that expected/a regression?

cc @samitolvanen @MaskRay

[samitolvanen]

No, that's not expected. You should see one process with multiple threads. Unless something changed and we now have to explicitly specify the number of ThinLTO jobs?

[torvic9]

Maybe related: I've never seen ThinLTO use all of the available CPUs when building the kernel (lld-15 and lld-16). At best, it uses exactly half; in my case, 12 out of 24 on a Zen2.

Is there some sort of a "limit" on how many threads LTO can use?

[torvic9]

I checked it again, Linux 6.3.3 + Clang16:

Linking vmlinux.o starts off with using half of the available CPUs, then quickly goes down to just using a single thread.

[samitolvanen]

You could try passing --thinlto-jobs=N to lld and see if that makes any difference?

https://clang.llvm.org/docs/ThinLTO.html#controlling-backend-parallelism

[nathanchance]

Could llvm/llvm-project@a8788de be related?