Re: [PATCH v7 4/6] perf record: Track sideband events for all CPUs when tracing selected CPUs

[Yang Jihong <yangjihong1@huawei.com>]

Hello,

On 2023/8/26 22:59, Namhyung Kim wrote:
> Hello,
> 
> On Fri, Aug 25, 2023 at 8:29 PM Yang Jihong <yangjihong1@huawei.com> 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@huawei.com>
>> Acked-by: Adrian Hunter <adrian.hunter@intel.com>
>> ---
>>   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 83bd1f117191..21c571018148 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)
>> +{
>> +       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.
> 
> Maybe I missed some earlier discussion but why is it not
> needed when exclude_user is set?  I think it still needs
> FORK or COMM at least..
> 

This is Adrian's suggestion earlier, I think it's probably because if 
exclude_user is set, MMAP information is not needed, that's my guess.

However, as you said, even if exclude_user is set, at least FORK and 
COMM are required.

Therefore, the conditions here need to be changed to:
"system_wide is need as long as there is a non-dummy event."

@Adrian, is this change okay?

Thanks,
Yang