Re: [PATCH v6 4/7] perf record: Track sideband events for all CPUs when tracing selected CPUs

[Yang Jihong]

Hello,

On 2023/8/25 13:17, Ian Rogers wrote:
> On Sun, Aug 20, 2023 at 6:30 PM Yang Jihong <yangjihong1@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Acked-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> > ---
> >   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 4ee94058028f..ae2e21b945fa 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)
>
> I think this would be better named something like:
> record__need_system_wide_dummy_event
Okay, it'll be modified in the next version.

Thanks,
Yang