libbpf: Add perf_buffer APIs for better integration with outside epoll loop

Add a set of APIs to perf_buffer manage to allow applications to integrate
perf buffer polling into existing epoll-based infrastructure. One example is
applications using libevent already and wanting to plug perf_buffer polling,
instead of relying on perf_buffer__poll() and waste an extra thread to do it.
But perf_buffer is still extremely useful to set up and consume perf buffer
rings even for such use cases.

So to accomodate such new use cases, add three new APIs:
  - perf_buffer__buffer_cnt() returns number of per-CPU buffers maintained by
    given instance of perf_buffer manager;
  - perf_buffer__buffer_fd() returns FD of perf_event corresponding to
    a specified per-CPU buffer; this FD is then polled independently;
  - perf_buffer__consume_buffer() consumes data from single per-CPU buffer,
    identified by its slot index.

To support a simpler, but less efficient, way to integrate perf_buffer into
external polling logic, also expose underlying epoll FD through
perf_buffer__epoll_fd() API. It will need to be followed by
perf_buffer__poll(), wasting extra syscall, or perf_buffer__consume(), wasting
CPU to iterate buffers with no data. But could be simpler and more convenient
for some cases.

These APIs allow for great flexiblity, but do not sacrifice general usability
of perf_buffer.

Also exercise and check new APIs in perf_buffer selftest.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20200821165927.849538-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -9373,6 +9373,11 @@ static int perf_buffer__process_records(struct perf_buffer *pb,
 	return 0;
 }
 
+int perf_buffer__epoll_fd(const struct perf_buffer *pb)
+{
+	return pb->epoll_fd;
+}
+
 int perf_buffer__poll(struct perf_buffer *pb, int timeout_ms)
 {
 	int i, cnt, err;
@@ -9390,6 +9395,55 @@ int perf_buffer__poll(struct perf_buffer *pb, int timeout_ms)
 	return cnt < 0 ? -errno : cnt;
 }
 
+/* Return number of PERF_EVENT_ARRAY map slots set up by this perf_buffer
+ * manager.
+ */
+size_t perf_buffer__buffer_cnt(const struct perf_buffer *pb)
+{
+	return pb->cpu_cnt;
+}
+
+/*
+ * Return perf_event FD of a ring buffer in *buf_idx* slot of
+ * PERF_EVENT_ARRAY BPF map. This FD can be polled for new data using
+ * select()/poll()/epoll() Linux syscalls.
+ */
+int perf_buffer__buffer_fd(const struct perf_buffer *pb, size_t buf_idx)
+{
+	struct perf_cpu_buf *cpu_buf;
+
+	if (buf_idx >= pb->cpu_cnt)
+		return -EINVAL;
+
+	cpu_buf = pb->cpu_bufs[buf_idx];
+	if (!cpu_buf)
+		return -ENOENT;
+
+	return cpu_buf->fd;
+}
+
+/*
+ * Consume data from perf ring buffer corresponding to slot *buf_idx* in
+ * PERF_EVENT_ARRAY BPF map without waiting/polling. If there is no data to
+ * consume, do nothing and return success.
+ * Returns:
+ *   - 0 on success;
+ *   - <0 on failure.
+ */
+int perf_buffer__consume_buffer(struct perf_buffer *pb, size_t buf_idx)
+{
+	struct perf_cpu_buf *cpu_buf;
+
+	if (buf_idx >= pb->cpu_cnt)
+		return -EINVAL;
+
+	cpu_buf = pb->cpu_bufs[buf_idx];
+	if (!cpu_buf)
+		return -ENOENT;
+
+	return perf_buffer__process_records(pb, cpu_buf);
+}
+
 int perf_buffer__consume(struct perf_buffer *pb)
 {
 	int i, err;
@@ -9402,7 +9456,7 @@ int perf_buffer__consume(struct perf_buffer *pb)
 
 		err = perf_buffer__process_records(pb, cpu_buf);
 		if (err) {
-			pr_warn("error while processing records: %d\n", err);
+			pr_warn("perf_buffer: failed to process records in buffer #%d: %d\n", i, err);
 			return err;
 		}
 	}

tools/lib/bpf/libbpf.h

@@ -588,8 +588,12 @@ perf_buffer__new_raw(int map_fd, size_t page_cnt,
 		     const struct perf_buffer_raw_opts *opts);
 
 LIBBPF_API void perf_buffer__free(struct perf_buffer *pb);
+LIBBPF_API int perf_buffer__epoll_fd(const struct perf_buffer *pb);
 LIBBPF_API int perf_buffer__poll(struct perf_buffer *pb, int timeout_ms);
 LIBBPF_API int perf_buffer__consume(struct perf_buffer *pb);
+LIBBPF_API int perf_buffer__consume_buffer(struct perf_buffer *pb, size_t buf_idx);
+LIBBPF_API size_t perf_buffer__buffer_cnt(const struct perf_buffer *pb);
+LIBBPF_API int perf_buffer__buffer_fd(const struct perf_buffer *pb, size_t buf_idx);
 
 typedef enum bpf_perf_event_ret
 	(*bpf_perf_event_print_t)(struct perf_event_header *hdr,

tools/lib/bpf/libbpf.map

@@ -299,3 +299,11 @@ LIBBPF_0.1.0 {
 		btf__set_fd;
 		btf__set_pointer_size;
 } LIBBPF_0.0.9;
+
+LIBBPF_0.2.0 {
+	global:
+		perf_buffer__buffer_cnt;
+		perf_buffer__buffer_fd;
+		perf_buffer__epoll_fd;
+		perf_buffer__consume_buffer;
+} LIBBPF_0.1.0;

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

@@ -7,6 +7,8 @@
 #include "test_perf_buffer.skel.h"
 #include "bpf/libbpf_internal.h"
 
+static int duration;
+
 /* AddressSanitizer sometimes crashes due to data dereference below, due to
  * this being mmap()'ed memory. Disable instrumentation with
  * no_sanitize_address attribute
@@ -24,13 +26,31 @@ static void on_sample(void *ctx, int cpu, void *data, __u32 size)
 	CPU_SET(cpu, cpu_seen);
 }
 
+int trigger_on_cpu(int cpu)
+{
+	cpu_set_t cpu_set;
+	int err;
+
+	CPU_ZERO(&cpu_set);
+	CPU_SET(cpu, &cpu_set);
+
+	err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
+	if (err && CHECK(err, "set_affinity", "cpu #%d, err %d\n", cpu, err))
+		return err;
+
+	usleep(1);
+
+	return 0;
+}
+
 void test_perf_buffer(void)
 {
-	int err, on_len, nr_on_cpus = 0,  nr_cpus, i, duration = 0;
+	int err, on_len, nr_on_cpus = 0, nr_cpus, i;
 	struct perf_buffer_opts pb_opts = {};
 	struct test_perf_buffer *skel;
-	cpu_set_t cpu_set, cpu_seen;
+	cpu_set_t cpu_seen;
 	struct perf_buffer *pb;
+	int last_fd = -1, fd;
 	bool *online;
 
 	nr_cpus = libbpf_num_possible_cpus();
@@ -63,6 +83,9 @@ void test_perf_buffer(void)
 	if (CHECK(IS_ERR(pb), "perf_buf__new", "err %ld\n", PTR_ERR(pb)))
 		goto out_close;
 
+	CHECK(perf_buffer__epoll_fd(pb) < 0, "epoll_fd",
+	      "bad fd: %d\n", perf_buffer__epoll_fd(pb));
+
 	/* trigger kprobe on every CPU */
 	CPU_ZERO(&cpu_seen);
 	for (i = 0; i < nr_cpus; i++) {
@@ -71,16 +94,8 @@ void test_perf_buffer(void)
 			continue;
 		}
 
-		CPU_ZERO(&cpu_set);
-		CPU_SET(i, &cpu_set);
-
-		err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set),
-					     &cpu_set);
-		if (err && CHECK(err, "set_affinity", "cpu #%d, err %d\n",
-				 i, err))
+		if (trigger_on_cpu(i))
 			goto out_close;
-
-		usleep(1);
 	}
 
 	/* read perf buffer */
@@ -92,6 +107,34 @@ void test_perf_buffer(void)
 		  "expect %d, seen %d\n", nr_on_cpus, CPU_COUNT(&cpu_seen)))
 		goto out_free_pb;
 
+	if (CHECK(perf_buffer__buffer_cnt(pb) != nr_cpus, "buf_cnt",
+		  "got %zu, expected %d\n", perf_buffer__buffer_cnt(pb), nr_cpus))
+		goto out_close;
+
+	for (i = 0; i < nr_cpus; i++) {
+		if (i >= on_len || !online[i])
+			continue;
+
+		fd = perf_buffer__buffer_fd(pb, i);
+		CHECK(fd < 0 || last_fd == fd, "fd_check", "last fd %d == fd %d\n", last_fd, fd);
+		last_fd = fd;
+
+		err = perf_buffer__consume_buffer(pb, i);
+		if (CHECK(err, "drain_buf", "cpu %d, err %d\n", i, err))
+			goto out_close;
+
+		CPU_CLR(i, &cpu_seen);
+		if (trigger_on_cpu(i))
+			goto out_close;
+
+		err = perf_buffer__consume_buffer(pb, i);
+		if (CHECK(err, "consume_buf", "cpu %d, err %d\n", i, err))
+			goto out_close;
+
+		if (CHECK(!CPU_ISSET(i, &cpu_seen), "cpu_seen", "cpu %d not seen\n", i))
+			goto out_close;
+	}
+
 out_free_pb:
 	perf_buffer__free(pb);
 out_close: