On Mon, Sep 30, 2019 at 1:32 AM Daniel Borkmann [off-list ref] wrote:

On 9/28/19 8:30 AM, Andrii Nakryiko wrote:

quoted

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.

quoted

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 <redacted>
---
  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.

quoted

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