Skip to content

Fix probe attachment failure when CPU 0 is offline #5479

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
Copilot wants to merge 2 commits into
base: master
Choose a base branch
from
Draft

Fix probe attachment failure when CPU 0 is offline #5479

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
af117e3
Initial plan
Copilot Mar 1, 2026
16b22c3
Fix hardcoded cpu=0 in libbpf.c: use get_first_online_cpu() for probe…
Copilot Mar 1, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
25 changes: 23 additions & 2 deletions src/cc/libbpf.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1080,6 +1080,27 @@ static int bpf_get_retprobe_bit(const char *event_type)
return errno ? -1 : ret;
}

/*
* Returns the first online CPU by parsing /sys/devices/system/cpu/online.
* The file contains a sorted list of ranges (e.g. "0-3", "1,3-5").
* Reading the first integer with fscanf stops at the first non-digit
* (hyphen or comma), which always gives the lowest-numbered online CPU.
* Falls back to CPU 0 if the file cannot be read or parsed.
*/
static int get_first_online_cpu(void)
{
FILE *fp;
int cpu = 0;

fp = fopen("/sys/devices/system/cpu/online", "r");
if (!fp)
return 0;
if (fscanf(fp, "%d", &cpu) != 1)
cpu = 0;
fclose(fp);
return cpu;
}

/*
* Kernel API with e12f03d ("perf/core: Implement the 'perf_kprobe' PMU") allows
* creating [k,u]probe with perf_event_open, which makes it easier to clean up
Expand All @@ -1092,7 +1113,7 @@ static int bpf_try_perf_event_open_with_probe(const char *name, uint64_t offs,
struct perf_event_attr attr = {};
int type = bpf_find_probe_type(event_type);
int is_return_bit = bpf_get_retprobe_bit(event_type);
int cpu = 0;
int cpu = get_first_online_cpu();

if (type < 0 || is_return_bit < 0)
return -1;
Expand Down Expand Up @@ -1160,7 +1181,7 @@ static const char *get_tracefs_path()
static int bpf_attach_tracing_event(int progfd, const char *event_path, int pid,
int *pfd)
{
int efd, cpu = 0;
int efd, cpu = get_first_online_cpu();
ssize_t bytes;
char buf[PATH_MAX];
struct perf_event_attr attr = {};
Expand Down