Re: [PATCH v5 bpf-next 2/2] selftests/bpf: add bpf_read_branch_records() selftest

[Yonghong Song <yhs@fb.com>]

On 1/25/20 8:52 PM, Alexei Starovoitov wrote:
> On Sun, Jan 26, 2020 at 04:50:14AM +0000, Yonghong Song wrote:
>>
>>
>> On 1/25/20 8:10 PM, Daniel Xu wrote:
>>> On Sat Jan 25, 2020 at 6:53 PM, Alexei Starovoitov wrote:
>>>> On Sat, Jan 25, 2020 at 2:32 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>>>>> +       attr.type = PERF_TYPE_HARDWARE;
>>>>> +       attr.config = PERF_COUNT_HW_CPU_CYCLES;
>>>>> +       attr.freq = 1;
>>>>> +       attr.sample_freq = 4000;
>>>>> +       attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
>>>>> +       attr.branch_sample_type = PERF_SAMPLE_BRANCH_USER | PERF_SAMPLE_BRANCH_ANY;
>>>>> +       pfd = syscall(__NR_perf_event_open, &attr, -1, 0, -1, PERF_FLAG_FD_CLOEXEC);
>>>>> +       if (CHECK(pfd < 0, "perf_event_open", "err %d\n", pfd))
>>>>> +               goto out_destroy;
>>>>
>>>>
>>>> It's failing for me in kvm. Is there way to make it work?
>>>> CIs will be vm based too. If this test requires physical host
>>>> such test will keep failing in all such environments.
>>>> Folks will be annoyed and eventually will disable the test.
>>>> Can we figure out how to test in the vm from the start?
>>>
>>> It seems there's a patchset that's adding LBR support to guest hosts:
>>> https://lkml.org/lkml/2019/8/6/215 . However it seems to be stuck in
>>> review limbo. Is there anything we can do to help that set along?
>>>
>>> As far as hacking it, nothing really comes to mind. Seems that patchset
>>> is our best hope.
>>
>> prog_tests/send_signal.c tests send_signal helper under nmi with
>> hardware counters. It added a check to see whether the underlying
>> hardware counter is supported, if it is not, the test is
>> skipped.
>>
>> Maybe we can use the same appraoch here. If perf_event_open with
>> PERF_TYPE_HARDWARE/PERF_SAMPLE_BRANCH_STACK failed,
>> we just mark the test as skipped instead of failing.
> 
> Instead of failing and skipping the test how about making it test error case?
> Like instead of lbr perf_event some other event can be passed into bpf prog.
> New helper can still be called and in such case it should return einval?

We can have both, I think. Some people may have a test environment
where PERF_SAMPLE_BRANCH_STACK is available, if there is a breakage,
then it will be reported.