On Sun, Sep 29, 2019 at 11:07 PM Song Liu [off-list ref] wrote:

quoted

On Sep 27, 2019, at 11:30 PM, Andrii Nakryiko [off-list ref] 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 <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";

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