Re: [PATCH v3 4/7] perf record: Track sideband events for all CPUs when tracing selected CPUs

[Yang Jihong <yangjihong1@huawei.com>]

Hello,

On 2023/7/31 19:08, Adrian Hunter wrote:
> On 22/07/23 12:32, 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_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>
>> ---
>>   tools/perf/Documentation/perf-record.txt |  3 +++
>>   tools/perf/builtin-record.c              | 14 +++++++++++++-
>>   2 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
>> index 680396c56bd1..dac53ece51ab 100644
>> --- a/tools/perf/Documentation/perf-record.txt
>> +++ b/tools/perf/Documentation/perf-record.txt
>> @@ -388,6 +388,9 @@ 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 3ff9d972225e..4e8e97928f05 100644
>> --- a/tools/perf/builtin-record.c
>> +++ b/tools/perf/builtin-record.c
>> @@ -912,6 +912,7 @@ 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;
>>   
>>   	/*
>> @@ -921,7 +922,18 @@ 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 all (non-dummy) evsel have exclude_user,
>> +		 * system_wide is not needed.
>> +		 */
>> +		if (!!opts->target.cpu_list && !opts->all_kernel)
> 
> Not everyone uses all-kernel.  Can we check the evsels are either dummy
> or exclude_user?
For perf_record, exclude_user of all evsels is set in evsel__config(), 
and record__config_tracking_events() is before evsel__config().

Uh..., it seems that only opts->all_kernel can be used to check 
exclude_user of evsels.

void evsel__config()
{
   ...
   if (opts->all_kernel) {
     attr->exclude_kernel = 0;
     attr->exclude_user   = 1;
   }
   ...
}

Thanks,
Yang