[PATCH bpf-next v2 1/1] selftests: bpf: size the map in test_lru_sanity3 to whole refills

[Eva Kurchatova <eva.kurchatova@virtuozzo.com>]

test_lru_sanity3 fills a map of tgt_free * 2 elements, and then reads
back all but the last few. Whether that passes depends on the cpu
count alone; on a machine with six it does not:

  test_lru_sanity3 (map_type:9 map_flags:0x0): test_lru_map.c:463:
  test_lru_sanity3: Assertion `!bpf_map_lookup_elem_with_ref_bit(
  lru_map_fd, key, value)' failed.

The kernel hands elements out in refills of lru->target_free, which is
clamp((size / nr_cpus) / 2, 1, LOCAL_FREE_TARGET), that is 128 / nr_cpus
rounded down. A refill the global free list cannot satisfy in full
calls __bpf_lru_list_shrink() for the remainder, and that evicts live
elements. Filling the map therefore evicts nothing only when computed
128 / nr_cpus is a power of two. Any other nr_cpus variation fails.

batch_size is already __tgt_size(tgt_free), the refill size of a map
is __map_size(batch_size) elements, so size the map that way and the
fill consumes whole refills. Start the keys of the last insert at
map_size + 1, just past the new size.

Above 128 cpus __tgt_size(tgt_free) is 0 and there is no size to build
on. Skip the test there.

Fixes: 5e9388f7984a ("selftests/bpf: adapt one more case in test_lru_map to the new target_free")
Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
---
 tools/testing/selftests/bpf/test_lru_map.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_lru_map.c b/tools/testing/selftests/bpf/test_lru_map.c
index 0921939532c6..84e9a930471a 100644
--- a/tools/testing/selftests/bpf/test_lru_map.c
+++ b/tools/testing/selftests/bpf/test_lru_map.c
@@ -441,8 +441,15 @@ static void test_lru_sanity3(int map_type, int map_flags, unsigned int tgt_free)
 	assert(sched_next_online(0, &next_cpu) != -1);
 
 	batch_size = __tgt_size(tgt_free);
+	if (!batch_size) {
+		/* More cpus than this tgt_free gives a refill for */
+		printf("Skip\n");
+		return;
+	}
+
+	/* A partial refill shrinks the LRU and evicts live elements */
+	map_size = __map_size(batch_size);
 
-	map_size = tgt_free * 2;
 	lru_map_fd = create_map(map_type, map_flags, map_size);
 	assert(lru_map_fd != -1);
 
@@ -466,7 +473,7 @@ static void test_lru_sanity3(int map_type, int map_flags, unsigned int tgt_free)
 	}
 
 	/* Insert new batch_size: replaces the non-referenced elements */
-	key = 2 * tgt_free + 1;
+	key = 1 + map_size;
 	end_key = key + batch_size;
 	for (; key < end_key; key++) {
 		assert(!bpf_map_update_elem(lru_map_fd, &key, value,
-- 
2.55.0