On Thu, Jan 16, 2020 at 1:05 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
>
> > On Wed, Jan 15, 2020 at 2:06 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> >>
> >> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
> >>
> >> > On Wed, Jan 15, 2020 at 6:13 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> >> >>
> >> >> From: Toke Høiland-Jørgensen <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?
>
> $ 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... :)
Ah, right, unconditional $(error), completely missed that few times, thanks!
>
> -Toke
>
