Re: [PATCH v2 4/7] perf record: Track sideband events for all CPUs when tracing selected CPUs

[Yang Jihong]

Hello,

On 2023/7/17 22:25, Adrian Hunter wrote:
> On 15/07/23 06:29, 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
> >      size                             136
> >      { sample_period, sample_freq }   4000
> >      sample_type                      IP|TID|TIME|ID|CPU|PERIOD
> >      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:HG
> >    ------------------------------------------------------------
> >    perf_event_attr:
> >      type                             1
> >      size                             136
> >      config                           0x9
> >      { 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>
> > ---
> >   tools/perf/Documentation/perf-record.txt | 3 +++
> >   tools/perf/builtin-record.c              | 7 ++++++-
> >   2 files changed, 9 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 ccbcb005e188..4a15b2e06f45 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -921,7 +921,12 @@ 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.
>
> So if all (non-dummy) evsel have exclude_user, then system_wide is not needed.
OK, The previous judgment system_wide is simplified and will be refined 
here.
> > +		 */
> > +		evsel = evlist__findnew_tracking_event(evlist, !!opts->target.cpu_list);
> >   		if (!evsel)
> >   			return -ENOMEM;
> >   
>
> .