selftests/bpf: Add ring_buffer__consume_n test.

Add a testcase for the ring_buffer__consume_n() API.

The test produces multiple samples in a ring buffer, using a
sys_getpid() fentry prog, and consumes them from user-space in batches,
rather than consuming all of them greedily, like ring_buffer__consume()
does.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/lkml/CAEf4BzaR4zqUpDmj44KNLdpJ=Tpa97GrvzuzVNO5nM6b7oWd1w@mail.gmail.com
Link: https://lore.kernel.org/bpf/20240425140627.112728-1-andrea.righi@canonical.com

tools/testing/selftests/bpf/Makefile

@@ -456,7 +456,7 @@ LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h		\
 LSKELS := fentry_test.c fexit_test.c fexit_sleep.c atomics.c 		\
 	trace_printk.c trace_vprintk.c map_ptr_kern.c 			\
 	core_kern.c core_kern_overflow.c test_ringbuf.c			\
-	test_ringbuf_map_key.c
+	test_ringbuf_n.c test_ringbuf_map_key.c
 
 # Generate both light skeleton and libbpf skeleton for these
 LSKELS_EXTRA := test_ksyms_module.c test_ksyms_weak.c kfunc_call_test.c \

tools/testing/selftests/bpf/prog_tests/ringbuf.c

@@ -13,6 +13,7 @@
 #include <linux/perf_event.h>
 #include <linux/ring_buffer.h>
 #include "test_ringbuf.lskel.h"
+#include "test_ringbuf_n.lskel.h"
 #include "test_ringbuf_map_key.lskel.h"
 
 #define EDONE 7777
@@ -326,6 +327,68 @@ cleanup:
 	test_ringbuf_lskel__destroy(skel);
 }
 
+/*
+ * Test ring_buffer__consume_n() by producing N_TOT_SAMPLES samples in the ring
+ * buffer, via getpid(), and consuming them in chunks of N_SAMPLES.
+ */
+#define N_TOT_SAMPLES	32
+#define N_SAMPLES	4
+
+/* Sample value to verify the callback validity */
+#define SAMPLE_VALUE	42L
+
+static int process_n_sample(void *ctx, void *data, size_t len)
+{
+	struct sample *s = data;
+
+	ASSERT_EQ(s->value, SAMPLE_VALUE, "sample_value");
+
+	return 0;
+}
+
+static void ringbuf_n_subtest(void)
+{
+	struct test_ringbuf_n_lskel *skel_n;
+	int err, i;
+
+	skel_n = test_ringbuf_n_lskel__open();
+	if (!ASSERT_OK_PTR(skel_n, "test_ringbuf_n_lskel__open"))
+		return;
+
+	skel_n->maps.ringbuf.max_entries = getpagesize();
+	skel_n->bss->pid = getpid();
+
+	err = test_ringbuf_n_lskel__load(skel_n);
+	if (!ASSERT_OK(err, "test_ringbuf_n_lskel__load"))
+		goto cleanup;
+
+	ringbuf = ring_buffer__new(skel_n->maps.ringbuf.map_fd,
+				   process_n_sample, NULL, NULL);
+	if (!ASSERT_OK_PTR(ringbuf, "ring_buffer__new"))
+		goto cleanup;
+
+	err = test_ringbuf_n_lskel__attach(skel_n);
+	if (!ASSERT_OK(err, "test_ringbuf_n_lskel__attach"))
+		goto cleanup_ringbuf;
+
+	/* Produce N_TOT_SAMPLES samples in the ring buffer by calling getpid() */
+	skel_n->bss->value = SAMPLE_VALUE;
+	for (i = 0; i < N_TOT_SAMPLES; i++)
+		syscall(__NR_getpgid);
+
+	/* Consume all samples from the ring buffer in batches of N_SAMPLES */
+	for (i = 0; i < N_TOT_SAMPLES; i += err) {
+		err = ring_buffer__consume_n(ringbuf, N_SAMPLES);
+		if (!ASSERT_EQ(err, N_SAMPLES, "rb_consume"))
+			goto cleanup_ringbuf;
+	}
+
+cleanup_ringbuf:
+	ring_buffer__free(ringbuf);
+cleanup:
+	test_ringbuf_n_lskel__destroy(skel_n);
+}
+
 static int process_map_key_sample(void *ctx, void *data, size_t len)
 {
 	struct sample *s;
@@ -384,6 +447,8 @@ void test_ringbuf(void)
 {
 	if (test__start_subtest("ringbuf"))
 		ringbuf_subtest();
+	if (test__start_subtest("ringbuf_n"))
+		ringbuf_n_subtest();
 	if (test__start_subtest("ringbuf_map_key"))
 		ringbuf_map_key_subtest();
 }

tools/testing/selftests/bpf/progs/test_ringbuf_n.c

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2024 Andrea Righi <andrea.righi@canonical.com>
+
+#include <linux/bpf.h>
+#include <sched.h>
+#include <unistd.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+#define TASK_COMM_LEN 16
+
+struct sample {
+	int pid;
+	long value;
+	char comm[16];
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_RINGBUF);
+} ringbuf SEC(".maps");
+
+int pid = 0;
+long value = 0;
+
+SEC("fentry/" SYS_PREFIX "sys_getpgid")
+int test_ringbuf_n(void *ctx)
+{
+	int cur_pid = bpf_get_current_pid_tgid() >> 32;
+	struct sample *sample;
+
+	if (cur_pid != pid)
+		return 0;
+
+	sample = bpf_ringbuf_reserve(&ringbuf, sizeof(*sample), 0);
+	if (!sample)
+		return 0;
+
+	sample->pid = pid;
+	sample->value = value;
+	bpf_get_current_comm(sample->comm, sizeof(sample->comm));
+
+	bpf_ringbuf_submit(sample, 0);
+
+	return 0;
+}