Re: [PATCH v8 bpf-next 3/3] bpf: add selftest for BPF_ENABLE_STATS

[Yonghong Song <yhs@fb.com>]

On 4/29/20 10:12 PM, Song Liu wrote:
> 
> 
>> On Apr 29, 2020, at 7:23 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>
>> On Tue, Apr 28, 2020 at 11:47 PM Song Liu <songliubraving@fb.com> wrote:
>>>
>>> Add test for BPF_ENABLE_STATS, which should enable run_time_ns stats.
>>>
>>> ~/selftests/bpf# ./test_progs -t enable_stats  -v
>>> test_enable_stats:PASS:skel_open_and_load 0 nsec
>>> test_enable_stats:PASS:get_stats_fd 0 nsec
>>> test_enable_stats:PASS:attach_raw_tp 0 nsec
>>> test_enable_stats:PASS:get_prog_info 0 nsec
>>> test_enable_stats:PASS:check_stats_enabled 0 nsec
>>> test_enable_stats:PASS:check_run_cnt_valid 0 nsec
>>> Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
>>>
>>> Signed-off-by: Song Liu <songliubraving@fb.com>
>>> ---
>>> .../selftests/bpf/prog_tests/enable_stats.c   | 46 +++++++++++++++++++
>>> .../selftests/bpf/progs/test_enable_stats.c   | 18 ++++++++
>>> 2 files changed, 64 insertions(+)
>>> create mode 100644 tools/testing/selftests/bpf/prog_tests/enable_stats.c
>>> create mode 100644 tools/testing/selftests/bpf/progs/test_enable_stats.c
>>>
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/enable_stats.c b/tools/testing/selftests/bpf/prog_tests/enable_stats.c
>>> new file mode 100644
>>> index 000000000000..cb5e34dcfd42
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/bpf/prog_tests/enable_stats.c
>>> @@ -0,0 +1,46 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +#include <test_progs.h>
>>> +#include <sys/mman.h>
>>
>> is this header used for anything?
> 
> Not really, will remove it.
> 
>>
>>> +#include "test_enable_stats.skel.h"
>>> +
>>> +void test_enable_stats(void)
>>> +{
>>
>> [...]
>>
>>> +
>>> +char _license[] SEC("license") = "GPL";
>>> +
>>> +static __u64 count;
>>
>> this is actually very unreliable, because compiler might decide to
>> just remove this variable. It should be either `static volatile`, or
>> better use zero-initialized global variable:
>>
>> __u64 count = 0;
> 
> Why would compile remove it? Is it because "static" or "no initialized?
> Would "__u64 count;" work?

It is because of "static". This static variable has file scope and the
compiler COULD remove count+=1 since it does not have any other effect
other than incrementting itself and nobody uses it.

> 
> For "__u64 count = 0;", checkpatch.pl generates an error:
> 
> ERROR: do not initialise globals to 0
> #92: FILE: tools/testing/selftests/bpf/progs/test_enable_stats.c:11:
> +__u64 count = 0;

I think this is okay.

For llvm10, you have to use `__u64 count = 0`.
For llvm11, you can use "__u64 count", the compiler changed global 
"common" variable treatment default from as a "common" var
to as a "bss" var.

In selftest, we have numerous cases for `__u64 count = 0` style
definitions and I recommend to use it as well since probably
quite some people uses llvm10 to compile/run selftests.

> 
> Thanks,
> Song
>