[PATCH v2 bpf-next] selftests/bpf: retry bpf_map_update_elem() when E2BIG is returned

[Matt Bobrowski <mattbobrowski@google.com>]

Executing the test_maps binary on platforms with extremely high core
counts may cause intermittent assertion failures in
test_update_delete() (called via test_map_parallel()). This can occur
because bpf_map_update_elem() under some circumstances (specifically
in this case while performing bpf_map_update_elem() with BPF_NOEXIST
on a BPF_MAP_TYPE_HASH with its map_flags set to BPF_F_NO_PREALLOC)
can return an E2BIG error code i.e.

error -7 7 tools/testing/selftests/bpf/test_maps.c:#: void
test_update_delete(unsigned int, void *): Assertion `err == 0' failed.
tools/testing/selftests/bpf/test_maps.c:#: void
__run_parallel(unsigned int, void (*)(unsigned int, void *), void *):
Assertion `status == 0' failed.

As it turns out, is_map_full() which is called from alloc_htab_elem()
can take on a conservative approach when htab->use_percpu_counter is
true (which is the case here because the percpu_counter is used when a
BPF_MAP_TYPE_HASH is created with its map_flags set to
BPF_F_NO_PREALLOC). This conservative approach prioritizes preventing
over-allocation and potential issues that could arise from possibly
exceeding htab->map.max_entries in highly concurrent environments,
even if it means slightly under-utilizing the htab map's capacity.

Given that bpf_map_update_elem() from test_update_delete() can return
E2BIG, update can_retry() such that it also accounts for the E2BIG
error code (specifically only when running with map_flags being set to
BPF_F_NO_PREALLOC). The retry loop will allow the global count
belonging to the percpu_counter to become synchronized and better
reflect the current htab map's capacity.

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Song Liu <song@kernel.org>
---
 tools/testing/selftests/bpf/test_maps.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index 3fae9ce46ca9..ccc5acd55ff9 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c
@@ -1399,7 +1399,8 @@ static void test_map_stress(void)
 static bool can_retry(int err)
 {
 	return (err == EAGAIN || err == EBUSY ||
-		(err == ENOMEM && map_opts.map_flags == BPF_F_NO_PREALLOC));
+		((err == ENOMEM || err == E2BIG) &&
+		 map_opts.map_flags == BPF_F_NO_PREALLOC));
 }
 
 int map_update_retriable(int map_fd, const void *key, const void *value, int flags, int attempts,
-- 
2.51.2.1041.gc1ab5b90ca-goog