Re: [PATCH bpf-next v2 02/10] tools/bpf/runqslower: Fix override option for VMLINUX_BTF

[Andrii Nakryiko]

On Wed, Jan 15, 2020 at 2:06 PM Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
>
> > On Wed, Jan 15, 2020 at 6:13 AM Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx> wrote:
> >>
> >> From: Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx>
> >>
> >> The runqslower tool refuses to build without a file to read vmlinux BTF
> >> from. The build fails with an error message to override the location by
> >> setting the VMLINUX_BTF variable if autodetection fails. However, the
> >> Makefile doesn't actually work with that override - the error message is
> >> still emitted.
> >
> > Do you have example command with VMLINUX_BTF override that didn't work
> > (and what error message was emitted)?
>
> Before this patch:
>
> $ cd ~/build/linux/tools/bpf/runqslower
> $ make
> Makefile:18: *** "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly".  Stop.
>
> $ make VMLINUX_BTF=~/build/linux/vmlinux
> Makefile:18: *** "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly".  Stop.

Ok, so this is strange. Try make clean and run with V=1, it might help
to debug this. This could happen if ~/build/linux/vmlinux doesn't
exist, but I assume you double-checked that. It works for me just fine
(Makefile won't do VMLINUX_BTF := assignment, if it's defined through
make invocation, so your change should be a no-op in that regard):

$ make clean
$ make VMLINUX_BTF=~/linux-build/default/vmlinux V=1
...
.output/sbin/bpftool btf dump file ~/linux-build/default/vmlinux
format c > .output/vmlinux.h
...

Wonder what your output looks like?

>
> >> Fix this by only doing auto-detection if no override is set. And while
> >> we're at it, also look for a vmlinux file in the current kernel build dir
> >> if none if found on the running kernel.
> >>
> >> Fixes: 9c01546d26d2 ("tools/bpf: Add runqslower tool to tools/bpf")
> >> Signed-off-by: Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx>
> >> ---
> >>  tools/bpf/runqslower/Makefile |   16 ++++++++++------
> >>  1 file changed, 10 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefile
> >> index cff2fbcd29a8..fb93ce2bf2fe 100644
> >> --- a/tools/bpf/runqslower/Makefile
> >> +++ b/tools/bpf/runqslower/Makefile
> >> @@ -10,12 +10,16 @@ CFLAGS := -g -Wall
> >>
> >>  # Try to detect best kernel BTF source
> >>  KERNEL_REL := $(shell uname -r)
> >> -ifneq ("$(wildcard /sys/kernel/btf/vmlinux)","")
> >> -VMLINUX_BTF := /sys/kernel/btf/vmlinux
> >> -else ifneq ("$(wildcard /boot/vmlinux-$(KERNEL_REL))","")
> >> -VMLINUX_BTF := /boot/vmlinux-$(KERNEL_REL)
> >> -else
> >> -$(error "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly")
> >> +ifeq ("$(VMLINUX_BTF)","")
> >> +  ifneq ("$(wildcard /sys/kernel/btf/vmlinux)","")
> >> +  VMLINUX_BTF := /sys/kernel/btf/vmlinux
> >> +  else ifneq ("$(wildcard /boot/vmlinux-$(KERNEL_REL))","")
> >> +  VMLINUX_BTF := /boot/vmlinux-$(KERNEL_REL)
> >> +  else ifneq ("$(wildcard $(abspath ../../../vmlinux))","")
> >> +  VMLINUX_BTF := $(abspath ../../../vmlinux)
> >
> > I'm planning to mirror runqslower into libbpf Github repo and this
> > ../../../vmlinux piece will be completely out of place in that
> > context. Also it only will help when building kernel in-tree. So I'd
> > rather not add this.
>
> Well building the kernel in-tree is something people sometimes want to do ;)
>
> Specifically, the selftests depend on this, so we should at least fix
> those; but I guess it could work to just pass in VMLINUX_BTF as part of
> the make -C from the selftests dir? I'll try that...

Yes, it can be handled through VMLINUX_BTF override for selftests. As
I said, this will be a self-contained example in libbpf's Github repo,
so this "in kernel tree" assumption doesn't stand there.


>
> -Toke
>