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

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Message-ID: <CAEf4Bzb7n+RGfKHP0ik7M6P7WGHke3FzsoLmuUmrEYmzK_Neog@mail.gmail.com>
Date:   Fri, 14 Feb 2020 09:47:43 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Xu <dxu@...uu.xyz>
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 Thu, Feb 13, 2020 at 11:05 PM Daniel Xu <dxu@...uu.xyz> wrote:
>
> 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.

Yeah, I meant get rid of perf_buffer and just use global variables for
outputting data.

re: validating multiple samples in perf_buffer. Given you don't really
control how many samples you'll get and you check nothing specific
about any single sample, I think it's fine to just validate any. They
are not supposed to differ much, right? Checking that size is multiple
of perf_branch_entry size is pretty much the only thing we can check,
no?

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