netdev - Re: [PATCH v2 bpf-next 2/3] libbpf: auto-set PERF_EVENT

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Message-ID: <CAEf4BzZuQoN5PZv+223OZZORhDNxx_ZK8pCgS2R3p=aTLFRfNw@mail.gmail.com>
Date:   Wed, 26 Jun 2019 15:31:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andriin@...com>, Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 2/3] libbpf: auto-set PERF_EVENT_ARRAY size to
 number of CPUs

On Wed, Jun 26, 2019 at 11:57 AM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jun 25, 2019, at 11:12 PM, Andrii Nakryiko <andriin@...com> wrote:
> >
> > For BPF_MAP_TYPE_PERF_EVENT_ARRAY typically correct size is number of
> > possible CPUs. This is impossible to specify at compilation time. This
> > change adds automatic setting of PERF_EVENT_ARRAY size to number of
> > system CPUs, unless non-zero size is specified explicitly. This allows
> > to adjust size for advanced specific cases, while providing convenient
> > and logical defaults.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > ---
> > tools/lib/bpf/libbpf.c | 17 ++++++++++++++++-
> > 1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index c74cc535902a..8f2b8a081ba7 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -2114,6 +2114,7 @@ static int
> > bpf_object__create_maps(struct bpf_object *obj)
> > {
> >       struct bpf_create_map_attr create_attr = {};
> > +     int nr_cpus = 0;
> >       unsigned int i;
> >       int err;
> >
> > @@ -2136,7 +2137,21 @@ bpf_object__create_maps(struct bpf_object *obj)
> >               create_attr.map_flags = def->map_flags;
> >               create_attr.key_size = def->key_size;
> >               create_attr.value_size = def->value_size;
> > -             create_attr.max_entries = def->max_entries;
> > +             if (def->type == BPF_MAP_TYPE_PERF_EVENT_ARRAY &&
> > +                 !def->max_entries) {
> > +                     if (!nr_cpus)
> > +                             nr_cpus = libbpf_num_possible_cpus();
> > +                     if (nr_cpus < 0) {
> > +                             pr_warning("failed to determine number of system CPUs: %d\n",
> > +                                        nr_cpus);
> > +                             return nr_cpus;
>
> I think we need to goto err_out here.

Absolutely, good catch, thanks!

>
> Thanks,
> Song
>