[PATCH v2] selftests: sched_ext: Add sched_ext as proper selftest target

[Mark Brown]

On Mon, Oct 07, 2024 at 09:31:32AM +0200, Björn Töpel wrote:

> When building the kselftest suite, e.g.:

>   make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- \
>     SKIP_TARGETS="" O=/output/foo -C tools/testing/selftests install

> The expectation is that the sched_ext is included, cross-built, and
> placed into /output/foo.

When building for arm64 with this applied on top of mainline or -next
I'm seeing:

   make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -C tools/testing/selftests TARGETS=sched_ext SKIP_TARGETS="

give

  CLNG-BPF create_dsq.bpf.o
In file included from create_dsq.bpf.c:9:
/home/broonie/git/linux/tools/sched_ext/include/scx/common.bpf.h:33:17: error: use of undeclared identifier 'SCX_DSQ_FLAG_BUILTIN'
   33 |         _Static_assert(SCX_DSQ_FLAG_BUILTIN,
      |                        ^

and more (my system clang is clang 20).  It's also failing with a native
x86_64 build in the same way.  I've run "make headers_install", it looks
like clang is not getting told about the copy of the headers installed
in ./usr/include:

clang -g -D__TARGET_ARCH_x86 -mlittle-endian -I/home/broonie/git/linux/tools/testing/selftests/sched_ext/include -I/home/broonie/git/linux/tools/testing/selftests/sched_ext/include/bpf-compat -I/home/broonie/git/linux/tools/testing/selftests/sched_ext/build/include -I/home/broonie/git/linux/tools/include/uapi -I/home/broonie/git/linux/tools/sched_ext/include -I/home/broonie/git/linux/include -idirafter /usr/lib/llvm-20/lib/clang/20/include -idirafter /usr/local/include -idirafter /usr/include/x86_64-linux-gnu -idirafter /usr/include  -Wall -Wno-compare-distinct-pointer-types -Wno-incompatible-function-pointer-types -O2 -mcpu=v3 -target bpf -c create_dsq.bpf.c -o /home/broonie/git/linux/tools/testing/selftests/sched_ext/build/obj/sched_ext/create_dsq.bpf.o

> Add CROSS_COMPILE, OUTPUT, and TARGETS support to the sched_ext
> selftest. Also, remove some variables that were unused by the
> Makefile.

> +ifneq ($(CROSS_COMPILE),)
> +DEFAULT_BPFTOOL := $(OUTPUT_DIR)/host/sbin/bpftool
> +HOST_OBJ_DIR := $(OBJ_DIR)/host/bpftool
> +HOST_LIBBPF_OUTPUT := $(OBJ_DIR)/host/libbpf/
> +HOST_LIBBPF_DESTDIR := $(OUTPUT_DIR)/host/
> +HOST_DESTDIR := $(OUTPUT_DIR)/host/
> +else
> +DEFAULT_BPFTOOL := $(OUTPUT_DIR)/sbin/bpftool
> +HOST_OBJ_DIR := $(OBJ_DIR)/bpftool
> +HOST_LIBBPF_OUTPUT := $(OBJ_DIR)/libbpf/
> +HOST_LIBBPF_DESTDIR := $(OUTPUT_DIR)/
> +HOST_DESTDIR := $(OUTPUT_DIR)/
> +endif

This won't detect a cross compile when building using LLVM as that
doesn't need to set CROSS_COMPILE, it can use the same binaries for all
targets.  There's runes in the Makefile for the mm selftests for
identifying the target architecture, though actually I'm wondering if
it's worth just always building the host copy and never having to worry
if the target build is a cross build or not?  It's obvious overhead in
the native case though if we actually need the target copy.
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