linux-kernel - Re: [PATCH v5 bpf-next 2/2] selftests/bpf: add bpf_read_branch

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Message-ID: <fcbc20dd-e5ca-e779-774f-49490dc87c3b@fb.com>
Date:   Sun, 26 Jan 2020 04:50:14 +0000
From:   Yonghong Song <yhs@...com>
To:     Daniel Xu <dxu@...uu.xyz>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>,
        "Andrii Nakryiko" <andriin@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v5 bpf-next 2/2] selftests/bpf: add
 bpf_read_branch_records() selftest



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@...uu.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.