Re: [PATCH 5/6] perf mem record: Request PERF_SAMPLE_CPU by default

[Ian Rogers]

On Fri, Sep 25, 2026 at 8:07 AM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
>
> The data type profiling per-sample stream keys cross-CPU contention on
> sample->cpu, which without PERF_SAMPLE_CPU is the "no CPU info"
> sentinel, making same-instance accesses from different cores
> indistinguishable from same-CPU traffic.  'perf mem record' already
> passes -d and -W to the record parser, add --sample-cpu and document it
> in perf-mem(1).
>
> Also grow rec_argv: the new entry overflows it on PMUs with separate
> load and store events, as the space for the fixed arguments was not
> reserved.
>
> Assisted-by: LLM
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks,
Ian

> ---
>  tools/perf/Documentation/perf-mem.txt | 4 ++++
>  tools/perf/builtin-mem.c              | 7 +++++--
>  2 files changed, 9 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..ade53688ebcd0f10 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 +135,9 @@ static int __cmd_record(int argc, const char **argv, struct perf_mem *mem,
>
>         rec_argv[i++] = "-d";
>
> +       /* Cross-CPU contention keys on sample->cpu, -1 without PERF_SAMPLE_CPU. */
> +       rec_argv[i++] = "--sample-cpu";
> +
>         if (mem->phys_addr)
>                 rec_argv[i++] = "--phys-data";
>
> --
> 2.53.0
>