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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>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
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Subject: Re: [PATCH bpf-next v3 02/11] tools/bpf/runqslower: Fix override option for VMLINUX_BTF
In-Reply-To: <CAEf4Bzbz47nFh4tPBn2PTi3+YiYpMDxymgf36P=ZjbuBPoCrZg@mail.gmail.com>
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Date:   Fri, 17 Jan 2020 10:44:52 +0100
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Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Thu, Jan 16, 2020 at 5:23 AM Toke H=C3=B8iland-J=C3=B8rgensen <toke@re=
dhat.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 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.
>>
>> Fix this by including the value of VMLINUX_BTF in the expansion, and only
>> emitting the error message if the *result* is empty. Also permit running
>> 'make clean' even though no VMLINUX_BTF is set.
>>
>> Fixes: 9c01546d26d2 ("tools/bpf: Add runqslower tool to tools/bpf")
>> Signed-off-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com>
>> ---
>>  tools/bpf/runqslower/Makefile |   18 ++++++++++--------
>>  1 file changed, 10 insertions(+), 8 deletions(-)
>>
>> diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefi=
le
>> index cff2fbcd29a8..89fb7cd30f1a 100644
>> --- a/tools/bpf/runqslower/Makefile
>> +++ b/tools/bpf/runqslower/Makefile
>> @@ -10,12 +10,14 @@ CFLAGS :=3D -g -Wall
>>
>>  # Try to detect best kernel BTF source
>>  KERNEL_REL :=3D $(shell uname -r)
>> -ifneq ("$(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")
>> +VMLINUX_BTF_PATHS :=3D $(VMLINUX_BTF) /sys/kernel/btf/vmlinux /boot/vml=
inux-$(KERNEL_REL)
>> +VMLINUX_BTF_PATH :=3D $(firstword $(wildcard $(VMLINUX_BTF_PATHS)))
>
> If user specifies VMLINUX_BTF pointing to non-existing file, but the
> system has /sys/kernel/btf/vmlinux, the latter will still be used,
> which is a very surprising behavior.

Hmm, yeah, good point.

> Also MAKECMDGOALS feels like a fragile hack to me. How about we move
> this VMLINUX_BTF guessing (without $(error)) into vmlinux.h rule
> itself and use shell if conditional after it to check for file
> existance and print nice error. That way we'll be checking VMLINUX_BTF
> only when it's really needed.

OK, sure, can do.

-Toke