selftests/bpf: Ensure that next_cpu() returns a valid CPU number

When using option -a without --prod-affinity or --cons-affinity, if the
number of producers and consumers is greater than the number of online
CPUs, the benchmark will fail to run as shown below:

  $ getconf _NPROCESSORS_ONLN
  8
  $ ./bench bpf-loop -a -p9
  Setting up benchmark 'bpf-loop'...
  setting affinity to CPU #8 failed: -22

Fix it by returning the remainder of next_cpu divided by the number of
online CPUs in next_cpu().

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230613080921.1623219-4-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bench.c

@@ -469,7 +469,7 @@ static int next_cpu(struct cpu_set *cpu_set)
 		exit(1);
 	}
 
-	return cpu_set->next_cpu++;
+	return cpu_set->next_cpu++ % env.nr_cpus;
 }
 
 static struct bench_state {
@@ -659,6 +659,7 @@ static void collect_measurements(long delta_ns) {
 
 int main(int argc, char **argv)
 {
+	env.nr_cpus = get_nprocs();
 	parse_cmdline_args_init(argc, argv);
 
 	if (env.list) {

tools/testing/selftests/bpf/bench.h

@@ -27,6 +27,7 @@ struct env {
 	bool quiet;
 	int consumer_cnt;
 	int producer_cnt;
+	int nr_cpus;
 	struct cpu_set prod_cpus;
 	struct cpu_set cons_cpus;
 };