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

[Yang Jihong <yangjihong1@huawei.com>]

Hello,

On 2023/8/28 19:25, Adrian Hunter wrote:
> On 28/08/23 06:03, Yang Jihong wrote:
>> 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?
> 
> If you wish.  I think we use FORK to get mappings, so not sure it
> would be needed.  There is PID, TID so COMM is not essential.
> There are FORK, COMM etc from the CPUs being traced of course.
> 
I think COMM is also necessary. If there is no COMM, the perf report 
cannot display the comm of the process migrated from other core.
The test result is as follows:

   # perf report --stdio
   # To display the perf.data header info, please use 
--header/--header-only options.
   #
   #
   # Total Lost Samples: 0
   #
   # Samples: 33  of event 'cpu-clock:khppp'
   # Event count (approx.): 8250000
   #
   # Overhead  Command          Shared Object      Symbol
   # ........  ...............  ................. 
.......................................
   #
       15.15%  test3            [kernel.kallsyms]  [k] finish_task_switch
        9.09%  kworker/1:0-eve  [kernel.kallsyms]  [k] _raw_spin_unlock_irq
        3.03%  :934             [kernel.kallsyms]  [k] blk_done_softirq
        3.03%  :934             [kernel.kallsyms]  [k] finish_task_switch
   <SNIP>

Thanks,
Yang