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Subject[PATCH 5.5 160/367] libbpf: Dont attach perf_buffer to offline/missing CPUs
Date
From: Andrii Nakryiko <andriin@fb.com>

commit 783b8f01f5942a786998f5577bd9ff3992f22a1a upstream.

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: fb84b8224655 ("libbpf: add perf buffer API")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191212013609.1691168-1-andriin@fb.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
tools/lib/bpf/libbpf.c | 32 +++++++++++++++++++++++++-------
1 file changed, 25 insertions(+), 7 deletions(-)

--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5945,7 +5945,7 @@ 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 */
};
@@ -6079,11 +6079,13 @@ perf_buffer__new_raw(int map_fd, size_t
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",
@@ -6152,20 +6154,32 @@ static struct perf_buffer *__perf_buffer
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);
@@ -6177,21 +6191,25 @@ static struct perf_buffer *__perf_buffer
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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