Commit 783b8f01 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
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libbpf: Don't attach perf_buffer to offline/missing CPUs 



It's quite common on some systems to have more CPUs enlisted as "possible",
than there are (and could ever be) present/online CPUs. In such cases,
perf_buffer creationg will fail due to inability to create perf event on
missing CPU with error like this:

libbpf: failed to open perf buffer event on cpu #16: No such device

This patch fixes the logic of perf_buffer__new() to ignore CPUs that are
missing or currently offline. In rare cases where user explicitly listed
specific CPUs to connect to, behavior is unchanged: libbpf will try to open
perf event buffer on specified CPU(s) anyways.

Fixes: fb84b822 ("libbpf: add perf buffer API")
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191212013609.1691168-1-andriin@fb.com
parent 65bc4c40

tools/lib/bpf/libbpf.c

+25 −7
Original line number Diff line number Diff line
@@ -5946,7 +5946,7 @@ struct perf_buffer { 
	size_t mmap_size;

	struct perf_cpu_buf **cpu_bufs;

	struct epoll_event *events;

	int cpu_cnt;

	int cpu_cnt; /* number of allocated CPU buffers */

	int epoll_fd; /* perf event FD */

	int map_fd; /* BPF_MAP_TYPE_PERF_EVENT_ARRAY BPF map FD */

};
@@ -6080,11 +6080,13 @@ perf_buffer__new_raw(int map_fd, size_t page_cnt, 
static struct perf_buffer *__perf_buffer__new(int map_fd, size_t page_cnt,

					      struct perf_buffer_params *p)

{

	const char *online_cpus_file = "/sys/devices/system/cpu/online";

	struct bpf_map_info map = {};

	char msg[STRERR_BUFSIZE];

	struct perf_buffer *pb;

	bool *online = NULL;

	__u32 map_info_len;

	int err, i;

	int err, i, j, n;



	if (page_cnt & (page_cnt - 1)) {

		pr_warn("page count should be power of two, but is %zu\n",
@@ -6153,20 +6155,32 @@ static struct perf_buffer *__perf_buffer__new(int map_fd, size_t page_cnt, 
		goto error;

	}



	for (i = 0; i < pb->cpu_cnt; i++) {

	err = parse_cpu_mask_file(online_cpus_file, &online, &n);

	if (err) {

		pr_warn("failed to get online CPU mask: %d\n", err);

		goto error;

	}



	for (i = 0, j = 0; i < pb->cpu_cnt; i++) {

		struct perf_cpu_buf *cpu_buf;

		int cpu, map_key;



		cpu = p->cpu_cnt > 0 ? p->cpus[i] : i;

		map_key = p->cpu_cnt > 0 ? p->map_keys[i] : i;



		/* in case user didn't explicitly requested particular CPUs to

		 * be attached to, skip offline/not present CPUs

		 */

		if (p->cpu_cnt <= 0 && (cpu >= n || !online[cpu]))

			continue;



		cpu_buf = perf_buffer__open_cpu_buf(pb, p->attr, cpu, map_key);

		if (IS_ERR(cpu_buf)) {

			err = PTR_ERR(cpu_buf);

			goto error;

		}



		pb->cpu_bufs[i] = cpu_buf;

		pb->cpu_bufs[j] = cpu_buf;



		err = bpf_map_update_elem(pb->map_fd, &map_key,

					  &cpu_buf->fd, 0);
@@ -6178,21 +6192,25 @@ static struct perf_buffer *__perf_buffer__new(int map_fd, size_t page_cnt, 
			goto error;

		}



		pb->events[i].events = EPOLLIN;

		pb->events[i].data.ptr = cpu_buf;

		pb->events[j].events = EPOLLIN;

		pb->events[j].data.ptr = cpu_buf;

		if (epoll_ctl(pb->epoll_fd, EPOLL_CTL_ADD, cpu_buf->fd,

			      &pb->events[i]) < 0) {

			      &pb->events[j]) < 0) {

			err = -errno;

			pr_warn("failed to epoll_ctl cpu #%d perf FD %d: %s\n",

				cpu, cpu_buf->fd,

				libbpf_strerror_r(err, msg, sizeof(msg)));

			goto error;

		}

		j++;

	}

	pb->cpu_cnt = j;

	free(online);



	return pb;



error:

	free(online);

	if (pb)

		perf_buffer__free(pb);

	return ERR_PTR(err);

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