linux-kernel - Re: [PATCH v7 bpf-next RESEND 2/2] selftests/bpf: add bpf_read_branch

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Message-Id: <C0LP269G4WO4.1Q4M8CK4K92SU@dlxu-fedora-R90QNFJV>
Date:   Thu, 13 Feb 2020 23:05:03 -0800
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Andrii Nakryiko" <andrii.nakryiko@...il.com>
Cc:     "bpf" <bpf@...r.kernel.org>, "Alexei Starovoitov" <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "Song Liu" <songliubraving@...com>, "Yonghong Song" <yhs@...com>,
        "Andrii Nakryiko" <andriin@...com>,
        "open list" <linux-kernel@...r.kernel.org>,
        "Kernel Team" <kernel-team@...com>,
        "Peter Ziljstra" <peterz@...radead.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>
Subject: Re: [PATCH v7 bpf-next RESEND 2/2] selftests/bpf: add
 bpf_read_branch_records() selftest

On Tue Feb 11, 2020 at 11:30 AM, Andrii Nakryiko wrote:
> On Mon, Feb 10, 2020 at 12:09 PM Daniel Xu <dxu@...uu.xyz> wrote:
[...]

> 
> > +       /* generate some branches on cpu 0 */
> > +       CPU_ZERO(&cpu_set);
> > +       CPU_SET(0, &cpu_set);
> > +       err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
> > +       if (CHECK(err, "set_affinity", "cpu #0, err %d\n", err))
> > +               goto out_free_pb;
> > +       /* spin the loop for a while (random high number) */
> > +       for (i = 0; i < 1000000; ++i)
> > +               ++j;
> > +
>
> 
> test_perf_branches__detach here?

Yeah, good idea.

[...]

> > +struct fake_perf_branch_entry {
> > +       __u64 _a;
> > +       __u64 _b;
> > +       __u64 _c;
> > +};
> > +
> > +struct output {
> > +       int required_size;
> > +       int written_stack;
> > +       int written_map;
> > +};
> > +
> > +struct {
> > +       __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
> > +       __uint(key_size, sizeof(int));
> > +       __uint(value_size, sizeof(int));
> > +} perf_buf_map SEC(".maps");
> > +
> > +typedef struct fake_perf_branch_entry fpbe_t[30];
> > +
> > +struct {
> > +       __uint(type, BPF_MAP_TYPE_ARRAY);
> > +       __uint(max_entries, 1);
> > +       __type(key, __u32);
> > +       __type(value, fpbe_t);
> > +} scratch_map SEC(".maps");
>
> 
> Can you please use global variables instead of array

If you mean `scratch_map`, sure.

> and perf_event_array?

Do you mean replace the perf ring buffer with global variables? I think
that would limit the number of samples we validate in userspace to a fixed
number. It might be better to validate as many as the system gives us.

Let me know what you think. I might be overthinking it.

> Would make BPF side clearer and userspace simpler.
> struct output member will just become variables.


Thanks,
Daniel