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

[Adrian Hunter <adrian.hunter@intel.com>]

On 31/07/23 15:38, Yang Jihong wrote:
> 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;
>   }
>   ...
> }

The parser updates attr in accordance with ":k" etc.  I guess 
opts->all_kernel or opts->all_user override that as well.