Re: Question to perf record command

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Fri, Sep 29, 2023 at 2:56 AM Thomas Richter <tmricht@linux.ibm.com> wrote:
>
> I have a question regarding command perf record.
>
> Command ./perf record -R -e cycles -vvv -C0 -- true
> sets up perf_event_open system call with CPU == 0 and PID == -1.
>
> According to the documentation, this applies to all processes on CPU 0
> and not just the process named true.
>
> Is this indended? I thought process true should be traced when running on
> CPU 0. Shouldn't the perf_event_open() system call in called with
> CPU set to 0 and PID set to PID of process true?

It's intended.  When you specify CPU targets (-a or -C options),
it'd profile ALL processes on those CPUs.  The command line
task just controls when it terminates the sampling.  For example,
we often use sleep command to profile all processes for the
specified time.

  $ perf record -a -- sleep 1

Thanks,
Namhyung

>
> Tracing output:
> # ./perf record -R -e cycles -vvv -C0 -- true
> DEBUGINFOD_URLS=
> nr_cblocks: 0
> affinity: SYS
> mmap flush: 1
> comp level: 0
> perf record opening and mmapping events
> Opening: cycles
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   { sample_period, sample_freq }   4000
>   sample_type                      IP|TID|TIME|ID|CPU|PERIOD|RAW
>   read_format                      ID|LOST
>   disabled                         1
>   inherit                          1
>   freq                             1
>   sample_id_all                    1
>   exclude_guest                    1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1  cpu 0  group_fd -1  flags 0x8 = 5
> ....
>
>
> When command invocation is
> # ./perf record -R -e cycles -vvv -- true
>
> the forked process named true (PID 13092) is traced on all CPUs:
> [root@a35lp67 perf]# ./perf record -R -e cycles -vvv  -- true
> DEBUGINFOD_URLS=
> nr_cblocks: 0
> affinity: SYS
> mmap flush: 1
> comp level: 0
> perf record opening and mmapping events
> Opening: cycles
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             136
>   config                           0 (PERF_COUNT_HW_CPU_CYCLES)
>   { sample_period, sample_freq }   4000
>   sample_type                      IP|TID|TIME|CPU|PERIOD|RAW
>   read_format                      ID|LOST
>   disabled                         1
>   inherit                          1
>   mmap                             1
>   comm                             1
>   freq                             1
>   enable_on_exec                   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 13092  cpu 0  group_fd -1  flags 0x8 = 5
> sys_perf_event_open: pid 13092  cpu 1  group_fd -1  flags 0x8 = 6
> sys_perf_event_open: pid 13092  cpu 2  group_fd -1  flags 0x8 = 7
> sys_perf_event_open: pid 13092  cpu 3  group_fd -1  flags 0x8 = 9
> sys_perf_event_open: pid 13092  cpu 4  group_fd -1  flags 0x8 = 10
> sys_perf_event_open: pid 13092  cpu 5  group_fd -1  flags 0x8 = 11
> sys_perf_event_open: pid 13092  cpu 6  group_fd -1  flags 0x8 = 12
> sys_perf_event_open: pid 13092  cpu 7  group_fd -1  flags 0x8 = 13
> mmap size 528384B
> ....
>
> --
> Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
> --
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