Re: [PATCH bpf] selftests/bpf: work-around EBUSY errors from hashmap update/delete

On Tue, Dec 22, 2020 at 3:58 PM Song Liu [off-list ref] wrote:

quoted

On Dec 22, 2020, at 11:53 AM, Andrii Nakryiko [off-list ref] wrote:

20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked") introduced
a possibility of getting EBUSY error on lock contention, which seems to happen
very deterministically in test_maps when running 1024 threads on low-CPU
machine. In libbpf CI case, it's a 2 CPU VM and it's hitting this 100% of the
time. Work around by retrying on EBUSY (and EAGAIN, while we are at it) after
a small sleep. sched_yield() is too agressive and fails even after 20 retries,
so I went with usleep(1) for backoff.

Also log actual error returned to make it easier to see what's going on.

Fixes: 20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked")
Cc: Song Liu <redacted>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Thanks for the fix!

Acked-by: Song Liu <redacted>

With one minor nitpick below

quoted

---
tools/testing/selftests/bpf/test_maps.c | 46 +++++++++++++++++++++----
1 file changed, 40 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index 0ad3e6305ff0..809004f4995f 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c

@@ -1312,22 +1312,56 @@ static void test_map_stress(void)

#define DO_UPDATE 1
#define DO_DELETE 0

[...]

quoted

+                             printf("error %d %d\n", err, errno);
+                     assert(err == 0);
+                     err = map_update_retriable(fd, &key, &value, BPF_EXIST, 20);
+                     if (err)
+                             printf("error %d %d\n", err, errno);
+                     assert(err == 0);
              } else {
-                     assert(bpf_map_delete_elem(fd, &key) == 0);
+                     err = map_delete_retriable(fd, &key, 5);

nit: Why 5 here vs. 20 above?

Forgot to update here. I'll make all of them the same, thanks.

quoted

+                     if (err)
+                             printf("error %d %d\n", err, errno);
+                     assert(err == 0);
              }
      }
}
--
2.24.1