Why don't you set Nix build cores to zero in your Nix config?

I haven't set it but previously it was using only -j1. Do you mean that if nix.buildCores is not set in configuration.nix, nixos assumes -j1 ?
I never paid attention before but it got really annoying with the kernel recompilations.

Seems confirmed with

if [ -z "${NIX_BUILD_CORES:-}" ]; then
  NIX_BUILD_CORES="1"
elif [ "$NIX_BUILD_CORES" -le 0 ]; then
  NIX_BUILD_CORES=$(nproc 2>/dev/null || true)
  if expr >/dev/null 2>&1 "$NIX_BUILD_CORES" : "^[0-9][0-9]*$"; then
    :
  else
    NIX_BUILD_CORES="1"
  fi
fi

I didn't check further because the pb looked similar to #25208
Let s try with buildCores ;)

I just tried with nix.buildCores set to 4 on nixos-unstable within a nix-shell and I don't know why but it dosn't work I get during the buildPhase:
build flags: -j1 -l1 SHELL=/nix/store/4ada72n7785wwazv42fhsnxjvilaa3aj-bash-4.4-p12/bin/bash O=\$\(buildRoot\) ARCH=x86_64 KBUILD_BUILD_VERSION=1-NixOS bzImage vmlinux modules KBUILD_BUILD_TIMESTAMP=jeu.\ janv.\ \ 1\ 00:00:01\ UTC\ 1970 KBUILD_BUILD_TIMESTAMP=jeu.\ janv.\ \ 1\ 00:00:01\ UTC\ 1970

EDIT: I guess it has to export NIX_BUILD_CORES else it will default to one.

You have to perform nixos-rebuild switch after setting nix.buildCores in the configuration.nix: Nix tools read the value from /etc/nix/nix.conf.

I had done it.

Please confirm that /etc/nix/nix.conf contains build-cores = 0. nix-shell '<nixpkgs>' -A linux --run 'echo $NIX_BUILD_CORES' should print the actual number of cores on the machine (without this PR). NIX_BUILD_CORES is set by Nix: https://nixos.org/nix/manual/#part-command-ref

arf seems like I had interrupted the rebuild because of a wrong channel my bad sorry. It works. I'll ditch the first commit, Is the 2nd commit of interest ?

I'll ditch the first commit, Is the 2nd commit of interest ?

Yes, please do.

Done manually.