Re: [PATCH 11/12] perf mem record: Request PERF_SAMPLE_CPU by default

[Namhyung Kim]

On Wed, Sep 16, 2026 at 08:47:38AM -0300, Arnaldo Carvalho de Melo wrote:
> From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 
> 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).
> 
> The rec_argv array is sized as nine arguments per memory PMU plus the
> user arguments, not counting the arguments __cmd_record() adds itself,
> up to eight with all the optional flags.  On PMUs with separate load and
> store events the four event arguments plus the fixed ones already filled
> the array to its last slot, so this new argument would write past it;
> reserve space for the fixed arguments explicitly.
> 
> Assisted-by: LLM
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>  tools/perf/Documentation/perf-mem.txt |  4 ++++
>  tools/perf/builtin-mem.c              | 18 ++++++++++++++++--
>  2 files changed, 20 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.

I guess the data source of samples can tell that on supported platforms.
Probably we can check TID for concurrent access, wdyt?

Thanks,
Namhyung

>  
>  "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..6f38cda1a45ada16 100644
> --- a/tools/perf/builtin-mem.c
> +++ b/tools/perf/builtin-mem.c
> @@ -99,8 +99,13 @@ 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 multiplied by number of PMUs that can
> +	 * support them, plus the arguments added directly below, at most:
> +	 * "record", "-W", "-d", "--sample-cpu", "--phys-data",
> +	 * "--data-page-size", "--all-user" and "--all-kernel".
> +	 */
> +	rec_argc = argc + 8 + 9 * (perf_pmu__mem_events_num_mem_pmus(pmu) + 1);
>  
>  	if (mem->cpu_list)
>  		rec_argc += 2;
> @@ -135,6 +140,15 @@ 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
>