[PATCH v2 8/8] perf mem record: Request PERF_SAMPLE_CPU by default

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

The data-type profiling per-sample stream keys cross-CPU contention on
sample->cpu: without PERF_SAMPLE_CPU the cpu field is the (u32)-1 "no
CPU info" sentinel, documented as such in perf_session__deliver_event(),
and same-instance reads and writes from different cores are
indistinguishable from same-CPU traffic, so pahole's false-sharing
detector cannot tell them apart.

builtin-record.c already defines --sample-cpu and 'perf mem record'
forwards unknown options to the record parser, so passing it explicitly
works today; make it the default, next to the -d (addr) and -W (weight)
the command already requests, documenting it in perf-mem(1).

Assisted-by: LLM
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/Documentation/perf-mem.txt | 4 ++++
 tools/perf/builtin-mem.c              | 9 +++++++++
 2 files changed, 13 insertions(+)

diff --git a/tools/perf/Documentation/perf-mem.txt b/tools/perf/Documentation/perf-mem.txt
index 4d164836d0943119..fe51c5e3333dc4a0 100644
--- a/tools/perf/Documentation/perf-mem.txt
+++ b/tools/perf/Documentation/perf-mem.txt
@@ -14,6 +14,10 @@ DESCRIPTION
 -----------
 "perf mem record" runs a command and gathers memory operation data
 from it, into perf.data. Perf record options are accepted and are passed through.
+It also requests the address (-d), the weight (-W, where supported) and the
+CPU id (--sample-cpu) of every sampled access by default; the CPU id is what
+lets per-sample analysis tell reads and writes to the same data from
+different cores apart from same-CPU traffic.
 
 "perf mem report" displays the result. It invokes perf report with the
 right set of options to display a memory access profile. By default, loads
diff --git a/tools/perf/builtin-mem.c b/tools/perf/builtin-mem.c
index 6101a26b3a781e69..a708e2549bae4ce7 100644
--- a/tools/perf/builtin-mem.c
+++ b/tools/perf/builtin-mem.c
@@ -135,6 +135,15 @@ static int __cmd_record(int argc, const char **argv, struct perf_mem *mem,
 
 	rec_argv[i++] = "-d";
 
+	/*
+	 * The data-type profiling per-sample stream keys cross-CPU
+	 * contention on sample->cpu (PERF_SAMPLE_CPU); without it the cpu
+	 * field is the (u32)-1 'no CPU info' sentinel and same-instance
+	 * reads and writes from different cores are indistinguishable
+	 * from same-CPU traffic.
+	 */
+	rec_argv[i++] = "--sample-cpu";
+
 	if (mem->phys_addr)
 		rec_argv[i++] = "--phys-data";
 
-- 
2.55.0