netdev - Re: [PATCH bpf-next] selftests: bpf: don't depend on hardcoded perf sample

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Date:   Tue, 26 Mar 2019 12:49:10 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH bpf-next] selftests: bpf: don't depend on hardcoded perf sample_freq

On Mon, Mar 25, 2019 at 9:46 AM Stanislav Fomichev <sdf@...ichev.me> wrote:
>
> On 03/25, Daniel Borkmann wrote:
> > On 03/19/2019 10:53 PM, Stanislav Fomichev wrote:
> > > When running stacktrace_build_id_nmi, try to query
> > > kernel.perf_event_max_sample_rate sysctl and use it as a sample_freq.
> > > If there was an error reading sysctl, fallback to 5000.
> > >
> > > kernel.perf_event_max_sample_rate sysctl can drift and/or can be
> > > adjusted by the perf tool, so assuming a fixed number might be
> > > problematic on a long running machine.
> > >
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> >
> > Mostly trying to understand rationale a bit better in context of
> > selftests; perf_event_max_sample_rate could drift also after this
> > patch here, but I presume you are saying that the frequency we
> > request below would interfere too much with perf tool adjusted
> > one and thus affect whole rest of kernel also after selftests
> > finished running, so below would handle it more gracefully, right?
> Not really, the kernel would straight out reject out attempt
> to syscall(perf_event_open) when sample_freq >=
> kernel.perf_event_max_sample_rate sysctl:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/events/core.c#n10724
>
> For this test, we don't really care about specific sample_freq, we just
> want our bpf prog to trigger at least once, so we can check the
> build-id.
>
> Maybe another way to fix it would be to convert to sample_period.
> Song, any specific reason you went with sample_freq and not
> sample_period in your original proposal?

I guess it's fine as-is.
Applied to bpf-next.
Thanks