Re: [PATCH bpf] libbpf: count present CPUs, not theoretically possible
From: Daniel Borkmann <daniel@iogearbox.net>
Date: 2019-09-30 08:32:10
Also in:
bpf
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.
quoted hunk ↗ jump to hunk
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.
int len = 0, n = 0, il = 0, ir = 0; unsigned int start = 0, end = 0; int tmp_cpus = 0;