[PATCH bpf-next 1/4] selftests/bpf: Fix cold_lru producing zero batch_hash in XDP LB benchmark

[Puranjay Mohan <puranjay@kernel.org>]

The cold_lru mechanism generates a per-batch hash to XOR into the source
address, ensuring each batch tests a fresh (uncached) flow:

    batch_hash = (batch_gen ^ cpu_id) * KNUTH_HASH_MULT;
    *saddr ^= batch_hash;

When batch_gen equals the CPU ID, the XOR is zero and batch_hash becomes
zero.  The source address is left unchanged, so every iteration hits the
warm LRU entry instead of testing cold lookups.

During validation, batch_gen is 2 (one increment from seeding, one from
the validation run itself).  If the BPF program happens to execute on
CPU 2, batch_hash is zero and all LRU lookups hit, producing:

    [udp-v4-lru-miss] COUNTER FAIL: LRU misses=0, expected 1
    Validation FAILED - aborting benchmark

This fails roughly 50% of the time depending on scheduler placement.

Fix by forcing the multiplier input to be non-zero with | 1.  Since
KNUTH_HASH_MULT (2654435761) is odd, the product of two odd numbers is
always odd, so batch_hash can never be zero.

Fixes: 4b4f2229104c ("selftests/bpf: Add XDP load-balancer BPF program")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 tools/testing/selftests/bpf/progs/xdp_lb_bench.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/xdp_lb_bench.c b/tools/testing/selftests/bpf/progs/xdp_lb_bench.c
index b9fd848c035d..f40e36aa183e 100644
--- a/tools/testing/selftests/bpf/progs/xdp_lb_bench.c
+++ b/tools/testing/selftests/bpf/progs/xdp_lb_bench.c
@@ -618,7 +618,7 @@ int xdp_lb_bench(struct xdp_md *xdp)
 		__u32 *saddr = data + saddr_off;
 
 		batch_gen++;
-		batch_hash = (batch_gen ^ bpf_get_smp_processor_id()) * KNUTH_HASH_MULT;
+		batch_hash = ((batch_gen ^ bpf_get_smp_processor_id()) | 1) * KNUTH_HASH_MULT;
 		if ((void *)(saddr + 1) <= data_end)
 			*saddr ^= batch_hash;
 	}
-- 
2.53.0-Meta