perf tests: Make "test data symbol" more robust on Neoverse N1

To prevent anyone from seeing a test failure appear as a regression and
thinking that it was caused by their code change, insert some noise into
the loop which makes it immune to sampling bias issues (errata 1694299).

The "test data symbol" test can fail with any unrelated change that
shifts the loop into an unfortunate position in the Perf binary which is
almost impossible to debug as the root cause of the test failure.
Ultimately it's caused by the referenced errata.

Fixes: 60abedb8 ("perf test: Introduce script for data symbol testing")
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: James Clark <james.clark@arm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Spoorthy S <spoorts2@in.ibm.com>
Link: https://lore.kernel.org/r/20240410103458.813656-2-james.clark@arm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/workloads/datasym.c

@@ -16,6 +16,22 @@ static int datasym(int argc __maybe_unused, const char **argv __maybe_unused)
 {
 	for (;;) {
 		buf1.data1++;
+		if (buf1.data1 == 123) {
+			/*
+			 * Add some 'noise' in the loop to work around errata
+			 * 1694299 on Arm N1.
+			 *
+			 * Bias exists in SPE sampling which can cause the load
+			 * and store instructions to be skipped entirely. This
+			 * comes and goes randomly depending on the offset the
+			 * linker places the datasym loop at in the Perf binary.
+			 * With an extra branch in the middle of the loop that
+			 * isn't always taken, the instruction stream is no
+			 * longer a continuous repeating pattern that interacts
+			 * badly with the bias.
+			 */
+			buf1.data1++;
+		}
 		buf1.data2 += buf1.data1;
 	}
 	return 0;