[PATCH bpf-next 2/4] selftests/bpf: Fix expired UDP LRU entries in XDP LB benchmark

[Puranjay Mohan <puranjay@kernel.org>]

populate_lru() inserts LRU entries with atime zero-initialized:

    struct real_pos_lru lru = { .pos = real_idx };

The BPF program's connection_table_lookup() enforces a 30-second timeout
for UDP flows:

    if (cur_time - dst_lru->atime > LRU_UDP_TIMEOUT)
        return NULL;

Since cur_time (system uptime) is always much larger than 30 seconds,
every pre-populated UDP entry is treated as expired on first access.
The lookup returns NULL, counting as an LRU miss.

This is masked by calibration: the BPF program runs during calibration
on one CPU, misses the expired entry, and re-inserts it with a fresh
atime.  But if the scheduler moves the thread to a different CPU for
validation, that CPU still has the atime=0 entry from populate_lru(),
causing intermittent validation failures for UDP LRU-hit scenarios:

    [udp-v4-lru-hit] COUNTER FAIL: LRU misses=1, expected 0

Fix by initializing atime with the current CLOCK_MONOTONIC time for UDP
flows, matching the clock source used by bpf_ktime_get_ns() in the BPF
program.

Fixes: a4b5ba8187cb ("selftests/bpf: Add XDP load-balancer benchmark driver")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 tools/testing/selftests/bpf/benchs/bench_xdp_lb.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/testing/selftests/bpf/benchs/bench_xdp_lb.c b/tools/testing/selftests/bpf/benchs/bench_xdp_lb.c
index 0b6709a2b03c..8e25bccbde92 100644
--- a/tools/testing/selftests/bpf/benchs/bench_xdp_lb.c
+++ b/tools/testing/selftests/bpf/benchs/bench_xdp_lb.c
@@ -563,12 +563,23 @@ static void create_per_cpu_lru_maps(struct xdp_lb_bench *skel)
 	nr_inner_maps = nr_cpus;
 }
 
+static __u64 ktime_get_ns(void)
+{
+	struct timespec ts;
+
+	clock_gettime(CLOCK_MONOTONIC, &ts);
+	return (__u64)ts.tv_sec * 1000000000ULL + ts.tv_nsec;
+}
+
 static void populate_lru(const struct test_scenario *sc, __u32 real_idx)
 {
 	struct real_pos_lru lru = { .pos = real_idx };
 	struct flow_key fk;
 	int i, err;
 
+	if (sc->ip_proto == IPPROTO_UDP)
+		lru.atime = ktime_get_ns();
+
 	build_flow_key(&fk, sc);
 
 	/* Insert into every per-CPU inner LRU so the entry is found
-- 
2.53.0-Meta