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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 16 Jan 2020 09:14:47 -0800
Message-ID: <CAEf4BzYLycZb+DBkao-jt+5sGgi3vbmzQ4Ogq9eRXZ+Jvew-ZA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 02/10] tools/bpf/runqslower: Fix override
 option for VMLINUX_BTF
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <jakub.kicinski@netronome.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
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On Thu, Jan 16, 2020 at 1:05 AM Toke H=C3=B8iland-J=C3=B8rgensen <toke@redh=
at.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Wed, Jan 15, 2020 at 2:06 PM Toke H=C3=B8iland-J=C3=B8rgensen <toke@=
redhat.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 <to=
ke@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=
 BTF
> >> >> from. The build fails with an error message to override the locatio=
n by
> >> >> setting the VMLINUX_BTF variable if autodetection fails. However, t=
he
> >> >> Makefile doesn't actually work with that override - the error messa=
ge is
> >> >> still emitted.
> >> >
> >> > Do you have example command with VMLINUX_BTF override that didn't wo=
rk
> >> > (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 hel=
p
> > 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 =
explicitly".  Stop.
> $ make VMLINUX_BTF=3D~/build/linux/vmlinux V=3D1
> 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 ("$(wildcard /sys/kernel/btf/vmlinux)","")
>   VMLINUX_BTF :=3D /sys/kernel/btf/vmlinux
>   else ifneq ("$(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 explici=
tly")
>   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
>