Avoid profiling offline or not present CPUs

Running example/c/profile on a virtual machine results in
libbpf_num_possible_cpus calculating the wrong number of CPUs, causing the
program to fail to set up performance monitor on a CPU/Core and throw an
error. In contrast, the Rust implementation ignores this error and continues
to execute.

This fix only applies to the C implementation.

Signed-off-by: ZhengSong Tu <[email protected]>

Author: tzssangglass

examples/c/profile.c

@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <string.h>
+#include <fcntl.h>
 #include <sys/syscall.h>
 #include <sys/sysinfo.h>
 #include <linux/perf_event.h>
@@ -13,6 +14,13 @@
 #include "profile.h"
 #include "blazesym.h"
 
+/*
+ * This function is from libbpf, but it is not a public API and can only be
+ * used for demonstration. We can use this here because we statically link
+ * against the libbpf built from submodule during build.
+ */
+extern int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz);
+
 static long perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
 			    int cpu, int group_fd, unsigned long flags)
 {
@@ -115,14 +123,16 @@ static void show_help(const char *progname)
 
 int main(int argc, char * const argv[])
 {
+	const char *online_cpus_file = "/sys/devices/system/cpu/online";
 	int freq = 1, pid = -1, cpu;
 	struct profile_bpf *skel = NULL;
 	struct perf_event_attr attr;
 	struct bpf_link **links = NULL;
 	struct ring_buffer *ring_buf = NULL;
-	int num_cpus;
+	int num_cpus, num_online_cpus;
 	int *pefds = NULL, pefd;
 	int argp, i, err = 0;
+	bool *online_mask = NULL;
 
 	while ((argp = getopt(argc, argv, "hf:")) != -1) {
 		switch (argp) {
@@ -139,16 +149,24 @@ int main(int argc, char * const argv[])
 		}
 	}
 
+	err = parse_cpu_mask_file(online_cpus_file, &online_mask, &num_online_cpus);
+	if (err) {
+		fprintf(stderr, "Fail to get online CPU numbers: %d\n", err);
+		goto cleanup;
+	}
+
 	num_cpus = libbpf_num_possible_cpus();
 	if (num_cpus <= 0) {
 		fprintf(stderr, "Fail to get the number of processors\n");
-		return 1;
+		err = -1;
+		goto cleanup;
 	}
 
 	skel = profile_bpf__open_and_load();
 	if (!skel) {
 		fprintf(stderr, "Fail to open and load BPF skeleton\n");
-		return 1;
+		err = -1;
+		goto cleanup;
 	}
 
 	symbolizer = blazesym_new();
@@ -166,8 +184,9 @@ int main(int argc, char * const argv[])
 	}
 
 	pefds = malloc(num_cpus * sizeof(int));
-	for (i = 0; i < num_cpus; i++)
+	for (i = 0; i < num_cpus; i++) {
 		pefds[i] = -1;
+	}
 
 	links = calloc(num_cpus, sizeof(struct bpf_link *));
 
@@ -179,10 +198,15 @@ int main(int argc, char * const argv[])
 	attr.freq = 1;
 
 	for (cpu = 0; cpu < num_cpus; cpu++) {
+		/* skip offline/not present CPUs */
+		if (cpu >= num_online_cpus || !online_mask[cpu])
+			continue;
+
 		/* Set up performance monitoring on a CPU/Core */
 		pefd = perf_event_open(&attr, pid, cpu, -1, PERF_FLAG_FD_CLOEXEC);
 		if (pefd < 0) {
 			fprintf(stderr, "Fail to set up performance monitor on a CPU/Core\n");
+			err = -1;
 			goto cleanup;
 		}
 		pefds[cpu] = pefd;
@@ -194,7 +218,7 @@ int main(int argc, char * const argv[])
 			goto cleanup;
 		}
 	}
-
+	
 	/* Wait and receive stack traces */
 	while (ring_buffer__poll(ring_buf, -1) >= 0) {
 	}
@@ -215,5 +239,6 @@ int main(int argc, char * const argv[])
 	ring_buffer__free(ring_buf);
 	profile_bpf__destroy(skel);
 	blazesym_free(symbolizer);
+	free(online_mask);
 	return -err;
 }