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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
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Subject: Re: [PATCH bpf-next v2 02/10] tools/bpf/runqslower: Fix override option for VMLINUX_BTF
In-Reply-To: <CAEf4BzZpGe-1S5_iwS8GBw9iiyFJmDUkOaO+2qaftRn_iy5cNA@mail.gmail.com>
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Date:   Thu, 16 Jan 2020 10:05:19 +0100
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Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Wed, Jan 15, 2020 at 2:06 PM Toke H=C3=B8iland-J=C3=B8rgensen <toke@re=
dhat.com> wrote:
>>
>> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>>
>> > On Wed, Jan 15, 2020 at 6:13 AM Toke H=C3=B8iland-J=C3=B8rgensen <toke=
@redhat.com> wrote:
>> >>
>> >> From: Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com>
>> >>
>> >> The runqslower tool refuses to build without a file to read vmlinux B=
TF
>> >> 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=3D~/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=3D1, 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 :=3D assignment, if it's defined through
> make invocation, so your change should be a no-op in that regard):
>
> $ make clean
> $ make VMLINUX_BTF=3D~/linux-build/default/vmlinux V=3D1
> ...
> .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 ex=
plicitly".  Stop.
$ make VMLINUX_BTF=3D~/build/linux/vmlinux V=3D1
Makefile:18: *** "Can't detect kernel BTF, use VMLINUX_BTF to specify it ex=
plicitly".  Stop.

Take another look at the relevant part of the makefile:

  ifneq ("$(wildcard /sys/kernel/btf/vmlinux)","")
  VMLINUX_BTF :=3D /sys/kernel/btf/vmlinux
  else ifneq ("$(wildcard /boot/vmlinux-$(KERNEL_REL))","")
  VMLINUX_BTF :=3D /boot/vmlinux-$(KERNEL_REL)
  else
  $(error "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitl=
y")
  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