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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 16 Jan 2020 10:05:19 +0100

Andrii Nakryiko <[email-masked]> writes:

> On Wed, Jan 15, 2020 at 2:06 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> Andrii Nakryiko <[email-masked]> writes:
>>
>> > On Wed, Jan 15, 2020 at 6:13 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>> >>
>> >> From: Toke Høiland-Jørgensen <[email-masked]>
>> >>
>> >> 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?

$ make clean
Makefile:18: *** "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly".  Stop.
$ make VMLINUX_BTF=~/build/linux/vmlinux V=1
Makefile:18: *** "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly".  Stop.

Take another look at the relevant part of the makefile:

  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")
  endif

That if/else doesn't actually consider the value of VMLINUX_BTF; so the
override only works if one of the files being considered by the
auto-detection actually exists... :)

-Toke