Re: [PATCH v6 4/7] perf record: Track sideband events for all CPUs when tracing selected CPUs

[Adrian Hunter]

On 25/08/23 08:17, Ian Rogers wrote:
> On Sun, Aug 20, 2023 at 6:30 PM Yang Jihong <yangjihong1@xxxxxxxxxx> 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_MMAP events of taskA.
>> Therefore, the comm and symbols of taskA cannot be parsed.
>>
>> The solution is to record sideband events for all CPUs when tracing
>> selected CPUs. Because this modifies the default behavior, add related
>> comments to the perf record man page.
>>
>> 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 (PERF_TYPE_SOFTWARE)
>>     size                             136
>>     config                           0 (PERF_COUNT_SW_CPU_CLOCK)
>>     { sample_period, sample_freq }   4000
>>     sample_type                      IP|TID|TIME|CPU|PERIOD|IDENTIFIER
>>     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:u
>>   ------------------------------------------------------------
>>   perf_event_attr:
>>     type                             1 (PERF_TYPE_SOFTWARE)
>>     size                             136
>>     config                           0x9 (PERF_COUNT_SW_DUMMY)
>>     { sample_period, sample_freq }   1
>>     sample_type                      IP|TID|TIME|CPU|IDENTIFIER
>>     read_format                      ID|LOST
>>     inherit                          1
>>     exclude_kernel                   1
>>     exclude_hv                       1
>>     mmap                             1
>>     comm                             1
>>     task                             1
>>     sample_id_all                    1
>>     exclude_guest                    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>
>> Acked-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>> ---
>>  tools/perf/Documentation/perf-record.txt |  3 ++
>>  tools/perf/builtin-record.c              | 44 +++++++++++++++++++++++-
>>  2 files changed, 46 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
>> index d5217be012d7..1889f66addf2 100644
>> --- a/tools/perf/Documentation/perf-record.txt
>> +++ b/tools/perf/Documentation/perf-record.txt
>> @@ -374,6 +374,9 @@ comma-separated list with no space: 0,1. Ranges of CPUs are specified with -: 0-
>>  In per-thread mode with inheritance mode on (default), samples are captured only when
>>  the thread executes on the designated CPUs. Default is to monitor all CPUs.
>>
>> +User space tasks can migrate between CPUs, so when tracing selected CPUs,
>> +a dummy event is created to track sideband for all CPUs.
>> +
>>  -B::
>>  --no-buildid::
>>  Do not save the build ids of binaries in the perf.data files. This skips
>> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
>> index 4ee94058028f..ae2e21b945fa 100644
>> --- a/tools/perf/builtin-record.c
>> +++ b/tools/perf/builtin-record.c
>> @@ -906,10 +906,44 @@ static int record__config_off_cpu(struct record *rec)
>>         return off_cpu_prepare(rec->evlist, &rec->opts.target, &rec->opts);
>>  }
>>
>> +static bool record__tracking_system_wide(struct record *rec)
> 
> I think this would be better named something like:
> record__need_system_wide_dummy_event

Please no.  The tracking event is a perf tool invention identified
by evsel->tracking and used in evsel__config(). Please don't
confuse it with dummy events.  Just because the tracking event
*might* be a dummy event does not mean a dummy event is the
tracking event.

> 
> Thanks,
> Ian
> 
>> +{
>> +       struct record_opts *opts = &rec->opts;
>> +       struct evlist *evlist = rec->evlist;
>> +       struct evsel *evsel;
>> +
>> +       /*
>> +        * If all (non-dummy) evsel have exclude_user,
>> +        * system_wide is not needed.
>> +        *
>> +        * all_kernel and all_user will overwrite exclude_kernel and
>> +        * exclude_user of attr in evsel__config(), here need to check
>> +        * all the three items.
>> +        *
>> +        * Sideband system wide if one of the following conditions is met:
>> +        *
>> +        *   - all_user is set, and there is a non-dummy event
>> +        *   - all_user and all_kernel are not set, and there is
>> +        *     a non-dummy event without exclude_user
>> +        */
>> +       if (opts->all_kernel)
>> +               return false;
>> +
>> +       evlist__for_each_entry(evlist, evsel) {
>> +               if (!evsel__is_dummy_event(evsel)) {
>> +                       if (opts->all_user || !evsel->core.attr.exclude_user)
>> +                               return true;
>> +               }
>> +       }
>> +
>> +       return false;
>> +}
>> +
>>  static int record__config_tracking_events(struct record *rec)
>>  {
>>         struct record_opts *opts = &rec->opts;
>>         struct evlist *evlist = rec->evlist;
>> +       bool system_wide = false;
>>         struct evsel *evsel;
>>
>>         /*
>> @@ -919,7 +953,15 @@ static int record__config_tracking_events(struct record *rec)
>>          */
>>         if (opts->target.initial_delay || target__has_cpu(&opts->target) ||
>>             perf_pmus__num_core_pmus() > 1) {
>> -               evsel = evlist__findnew_tracking_event(evlist, false);
>> +
>> +               /*
>> +                * User space tasks can migrate between CPUs, so when tracing
>> +                * selected CPUs, sideband for all CPUs is still needed.
>> +                */
>> +               if (!!opts->target.cpu_list && record__tracking_system_wide(rec))
>> +                       system_wide = true;
>> +
>> +               evsel = evlist__findnew_tracking_event(evlist, system_wide);
>>                 if (!evsel)
>>                         return -ENOMEM;
>>
>> --
>> 2.30.GIT
>>