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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sun, Sep 29, 2019 at 11:07 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Sep 27, 2019, at 11:30 PM, Andrii Nakryiko <andriin@fb.com> 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.
> >
> > 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@fb.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";
>
> This is _very_ confusing. "possible cpus", "present cpus", and "online
> cpus" are existing terminologies. I don't think we should force people
> to remember something like "By possible cpus, libbpf actually means
> present cpus".
>
> This change works if we call it "libbbpf_num_cpus()". However,
> libbpf_num_possible_cpus(), should mean possible CPUs.

Ok, then if we really need to (I'll play again with my VM to recall
all the details of original problem with this that I had before), I'll
just add libbpf_num_present_cpus().

>
> Thanks,
> Song
>