[PATCH v3 6/7] 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 that field is the (u32)-1 "no CPU
info" sentinel, so same-instance accesses from different cores are
indistinguishable from same-CPU traffic.  'perf mem record' already
passes -d (addr) and -W (weight) explicitly to the record parser, add
--sample-cpu as well and document it in perf-mem(1).

The rec_argv array only had room for nine arguments per PMU plus the
user arguments, not counting the up to eight __cmd_record() adds itself,
which a new argument would overflow on PMUs with separate load and store
events; reserve space for the fixed arguments too.

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              | 11 +++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

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..f25592bc52728655 100644
--- a/tools/perf/builtin-mem.c
+++ b/tools/perf/builtin-mem.c
@@ -99,8 +99,8 @@ static int __cmd_record(int argc, const char **argv, struct perf_mem *mem,
 	argc = parse_options(argc, argv, options, record_usage,
 			     PARSE_OPT_KEEP_UNKNOWN);
 
-	/* Max number of arguments multiplied by number of PMUs that can support them. */
-	rec_argc = argc + 9 * (perf_pmu__mem_events_num_mem_pmus(pmu) + 1);
+	/* Max number of arguments per PMU plus the fixed ones added below. */
+	rec_argc = argc + 8 + 9 * (perf_pmu__mem_events_num_mem_pmus(pmu) + 1);
 
 	if (mem->cpu_list)
 		rec_argc += 2;
@@ -135,6 +135,13 @@ 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; without PERF_SAMPLE_CPU it is the (u32)-1 'no CPU
+	 * info' sentinel.
+	 */
+	rec_argv[i++] = "--sample-cpu";
+
 	if (mem->phys_addr)
 		rec_argv[i++] = "--phys-data";
 
-- 
2.55.0