6.6-stable review patch. If anyone has any objections, please let me know.
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From: James Clark james.clark@arm.com
[ Upstream commit 256ef072b3842273ce703db18b603b051aca95fe ]
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: 60abedb8aa902b06 ("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.com Signed-off-by: Arnaldo Carvalho de Melo acme@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- tools/perf/tests/workloads/datasym.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)
diff --git a/tools/perf/tests/workloads/datasym.c b/tools/perf/tests/workloads/datasym.c index ddd40bc63448a..8e08fc75a973e 100644 --- a/tools/perf/tests/workloads/datasym.c +++ b/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;