[PATCH bpf-next 3/4] selftests/bpf: Cap batch-timing calibration at BPF may_goto loop limit

[Puranjay Mohan <puranjay@kernel.org>]

BENCH_BPF_LOOP uses the may_goto instruction (via can_loop) to bound
its iteration loop.  The kernel enforces a maximum of BPF_MAX_LOOPS
(8,388,608) iterations per may_goto instruction.

CALIBRATE_MAX_BATCH was set to 10,000,000, exceeding this limit.  If
calibration computed a batch_iters above 8M, the BPF loop would exit
early when can_loop becomes false.  The timing sample would cover fewer
iterations than batch_iters, causing the per-op calculation to
underestimate the true cost.  Validation would also fail since map
counters would not reach the expected batch_iters + 1.

No current benchmark triggers this (the fastest, bpf-nop at ~1.8 ns/op,
calibrates to ~5.5M), but future sub-1.25 ns/op benchmarks would.

Lower the cap to 8,000,000 to stay within the may_goto limit.

Fixes: 08158c111d7d ("selftests/bpf: Add BPF batch-timing library")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 tools/testing/selftests/bpf/benchs/bench_bpf_timing.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c b/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c
index 75a39da69655..081e2e860cb4 100644
--- a/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c
+++ b/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c
@@ -166,7 +166,7 @@ void bpf_bench_timing_report(struct bpf_bench_timing *t, const char *name, const
 
 #define CALIBRATE_SEED_BATCH	100
 #define CALIBRATE_MIN_BATCH	100
-#define CALIBRATE_MAX_BATCH	10000000
+#define CALIBRATE_MAX_BATCH	8000000
 #define CALIBRATE_TARGET_MS	10
 #define CALIBRATE_RUNS		5
 #define PROPORTIONALITY_TOL	0.05	/* 5% */
-- 
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