Re: [PATCH 3/5] perf record: Tracking side-band events for all CPUs when tracing selected CPUs

[Adrian Hunter]

On 4/07/23 10:42, Yang Jihong wrote:
> User space tasks can migrate between CPUs, we need to track side-band
> events for all CPUs.
> 
> The specific scenarios are as follows:
> 
>          CPU0                                 CPU1
>   perf record -C 0 start
>                               taskA starts to be created and executed
>                                 -> PERF_RECORD_COMM and PERF_RECORD_MMAP
>                                    events only deliver to CPU1
>                               ......
>                                 |
>                           migrate to CPU0
>                                 |
>   Running on CPU0    <----------/
>   ...
> 
>   perf record -C 0 stop
> 
> Now perf samples the PC of taskA. However, perf does not record the
> PERF_RECORD_COMM and PERF_RECORD_COMM events of taskA.
> Therefore, the comm and symbols of taskA cannot be parsed.
> 
> The sys_perf_event_open invoked is as follows:
> 
>   # perf --debug verbose=3 record -e cpu-clock -C 1 true
>   <SNIP>
>   Opening: cpu-clock
>   ------------------------------------------------------------
>   perf_event_attr:
>     type                             1
>     size                             136
>     { sample_period, sample_freq }   4000
>     sample_type                      IP|TID|TIME|ID|CPU|PERIOD
>     read_format                      ID|LOST
>     disabled                         1
>     inherit                          1
>     freq                             1
>     sample_id_all                    1
>     exclude_guest                    1
>   ------------------------------------------------------------
>   sys_perf_event_open: pid -1  cpu 1  group_fd -1  flags 0x8 = 5
>   Opening: dummy:HG
>   ------------------------------------------------------------
>   perf_event_attr:
>     type                             1
>     size                             136
>     config                           0x9
>     { sample_period, sample_freq }   4000
>     sample_type                      IP|TID|TIME|ID|CPU|PERIOD
>     read_format                      ID|LOST
>     inherit                          1
>     mmap                             1
>     comm                             1
>     freq                             1
>     task                             1
>     sample_id_all                    1
>     mmap2                            1
>     comm_exec                        1
>     ksymbol                          1
>     bpf_event                        1
>   ------------------------------------------------------------
>   sys_perf_event_open: pid -1  cpu 0  group_fd -1  flags 0x8 = 6
>   sys_perf_event_open: pid -1  cpu 1  group_fd -1  flags 0x8 = 7
>   sys_perf_event_open: pid -1  cpu 2  group_fd -1  flags 0x8 = 9
>   sys_perf_event_open: pid -1  cpu 3  group_fd -1  flags 0x8 = 10
>   sys_perf_event_open: pid -1  cpu 4  group_fd -1  flags 0x8 = 11
>   sys_perf_event_open: pid -1  cpu 5  group_fd -1  flags 0x8 = 12
>   sys_perf_event_open: pid -1  cpu 6  group_fd -1  flags 0x8 = 13
>   sys_perf_event_open: pid -1  cpu 7  group_fd -1  flags 0x8 = 14
>   <SNIP>
> 
> Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> ---
>  tools/perf/builtin-record.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 8872cd037f2c..69e0d8c75aab 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -908,6 +908,31 @@ static int record__config_off_cpu(struct record *rec)
>  	return off_cpu_prepare(rec->evlist, &rec->opts.target, &rec->opts);
>  }
>  
> +static int record__config_tracking_events(struct record *rec)
> +{
> +	struct evsel *evsel;
> +	struct evlist *evlist = rec->evlist;
> +	struct record_opts *opts = &rec->opts;
> +
> +	/*
> +	 * User space tasks can migrate between CPUs, so when tracing
> +	 * selected CPUs, sideband for all CPUs is still needed.
> +	 */
> +	if (opts->target.cpu_list) {

I am not sure if anyone minds doing this by default, but perhaps
we should say something about it on the perf record man page.

> +		evsel = evlist__findnew_tracking_event(evlist);
> +		if (!evsel)
> +			return -ENOMEM;
> +
> +		if (!evsel->core.system_wide) {
> +			evsel->core.system_wide = true;
> +			evsel__set_sample_bit(evsel, TIME);
> +			perf_evlist__propagate_maps(&evlist->core, &evsel->core);
> +		}

Perhaps better to export via internel/evsel.h

void perf_evsel__go_system_wide(struct perf_evlist *evlist, struct perf_evsel *evsel)
{
	if (!evsel->system_wide) {
		evsel->system_wide = true;
		if (evlist->needs_map_propagation)
			__perf_evlist__propagate_maps(evlist, evsel);
	}
}

As suggested in response to patch 2, perhaps deal with system_wide
inside evlist__findnew_tracking_event()

> +	}
> +
> +	return 0;
> +}
> +
>  static bool record__kcore_readable(struct machine *machine)
>  {
>  	char kcore[PATH_MAX];
> @@ -4235,6 +4260,12 @@ int cmd_record(int argc, const char **argv)
>  		goto out;
>  	}
>  
> +	err = record__config_tracking_events(rec);
> +	if (err) {
> +		pr_err("record__config_tracking_events failed, error %d\n", err);
> +		goto out;
> +	}
> +
>  	err = record__init_thread_masks(rec);
>  	if (err) {
>  		pr_err("Failed to initialize parallel data streaming masks\n");