netdev - Re: [PATCH bpf] libbpf: count present CPUs, not theoretically possible

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Date:   Mon, 30 Sep 2019 09:26:47 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf] libbpf: count present CPUs, not theoretically possible

On Mon, Sep 30, 2019 at 1:32 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 9/28/19 8:30 AM, Andrii Nakryiko wrote:
> > This patch switches libbpf_num_possible_cpus() from using possible CPU
> > set to present CPU set. This fixes issues with incorrect auto-sizing of
> > PERF_EVENT_ARRAY map on HOTPLUG-enabled systems.
>
> Those issues should be described in more detail here in the changelog,
> otherwise noone knows what is meant exactly when glancing at the git log.

Sure, I can add more details.

>
> > On HOTPLUG enabled systems, /sys/devices/system/cpu/possible is going to
> > be a set of any representable (i.e., potentially possible) CPU, which is
> > normally way higher than real amount of CPUs (e.g., 0-127 on VM I've
> > tested on, while there were just two CPU cores actually present).
> > /sys/devices/system/cpu/present, on the other hand, will only contain
> > CPUs that are physically present in the system (even if not online yet),
> > which is what we really want, especially when creating per-CPU maps or
> > perf events.
> >
> > On systems with HOTPLUG disabled, present and possible are identical, so
> > there is no change of behavior there.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > ---
> >   tools/lib/bpf/libbpf.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index e0276520171b..45351c074e45 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -5899,7 +5899,7 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
> >
> >   int libbpf_num_possible_cpus(void)
> >   {
> > -     static const char *fcpu = "/sys/devices/system/cpu/possible";
> > +     static const char *fcpu = "/sys/devices/system/cpu/present";
>
> Problem is that this is going to break things *badly* for per-cpu maps as
> BPF_DECLARE_PERCPU() relies on possible CPUs, not present ones. And given
> present<=possible you'll end up corrupting user space when you do a lookup
> on the map since kernel side operates on possible as well.

Yeah, you are right. Ok, let me go back to my VM and repro original
issue I had and see what and why is causing that. I'll see maybe I
don't need this fix at all.

>
> >       int len = 0, n = 0, il = 0, ir = 0;
> >       unsigned int start = 0, end = 0;
> >       int tmp_cpus = 0;
> >
>