[PATCH bpf-next 4/4] selftests/bpf: Filter timing outliers with IQR in batch-timing library

[Puranjay Mohan <puranjay@kernel.org>]

System noise (timer interrupts, scheduling) can produce outlier batch
samples that inflate the reported stddev.  For example, tcp-v4-syn
showed stddev 37.86 ns without filtering vs 0.16 ns with filtering on
the same run, because a handful of interrupt-hit batches dominated the
variance.

Apply IQR-based outlier filtering (1.5 * IQR fences) before computing
statistics.  Samples outside [Q1 - 1.5*IQR, Q3 + 1.5*IQR] are
discarded.  This removes system noise while preserving genuine
operational variance: scenarios with inherently wide distributions
(e.g., LRU-miss with eviction pressure) have large IQR, so their
fences are wide and the filter has minimal effect.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 .../selftests/bpf/benchs/bench_bpf_timing.c   | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c b/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c
index 081e2e860cb4..1c3a74807c1c 100644
--- a/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c
+++ b/tools/testing/selftests/bpf/benchs/bench_bpf_timing.c
@@ -65,6 +65,31 @@ static int collect_samples(struct bpf_bench_timing *t,
 	return total;
 }
 
+static int filter_outliers_iqr(double *sorted, int n)
+{
+	double q1, q3, iqr, lo, hi;
+	int start = 0, end = n;
+
+	if (n < 8)
+		return n;
+
+	q1 = sorted[n / 4];
+	q3 = sorted[3 * n / 4];
+	iqr = q3 - q1;
+	lo = q1 - 1.5 * iqr;
+	hi = q3 + 1.5 * iqr;
+
+	while (start < end && sorted[start] < lo)
+		start++;
+	while (end > start && sorted[end - 1] > hi)
+		end--;
+
+	if (start > 0)
+		memmove(sorted, sorted + start, (end - start) * sizeof(double));
+
+	return end - start;
+}
+
 static void compute_stats(const double *sorted, int n,
 			  struct timing_stats *s)
 {
@@ -150,6 +175,7 @@ void bpf_bench_timing_report(struct bpf_bench_timing *t, const char *name, const
 		return;
 	}
 
+	total = filter_outliers_iqr(all, total);
 	compute_stats(all, total, &s);
 
 	if (t->machine_readable) {
-- 
2.53.0-Meta