Re: [PATCH V1 13/23] perf evlist: Add evlist__add_system_wide_dummy()

[Adrian Hunter]

On 6/05/22 11:34, Ian Rogers wrote:
> On Fri, May 6, 2022 at 1:14 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>
>> On 6/05/22 02:56, Ian Rogers wrote:
>>> On Thu, May 5, 2022 at 9:57 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>>>
>>>> Add evlist__add_system_wide_dummy() to enable creating a system-wide dummy
>>>> event that sets up the system-wide maps before map propagation.
>>>
>>> Perhaps this should be:
>>> evlist__add_dummy_on_all_online_cpus()
>>
>> Ok, but offline isn't an option, so I'll drop 'online' from the name.
> 
> I'm not sure I follow. There can be CPUs that are offline (echo 0 >
> /sys/devices/system/cpu/cpu2/online) and then brought online while
> perf is running. The output obviously won't be right as we're missing
> dummy events. Losing online from the name here seems to remove a
> useful clue.

perf_event_open() will not accept events on offline CPUs.

> 
> Thanks,
> Ian
> 
>>> my thoughts being that a system wide dummy sounds like the dummy CPU
>>> map, ie {-1} whilst what happens here is opening on all online CPUs
>>> {0-35} on a 36 hyperthread system. Note also that the libperf
>>> cpu_map__read_all_cpu_map code doesn't discover sysfs' mount properly,
>>> as done in:
>>> tools/lib/api/fs/fs.h
>>> Some tech debt.
>>>
>>>> For convenience, add evlist__add_aux_dummy() so that the logic can be used
>>>> whether or not the event needs to be system-wide.
>>>>
>>>> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>>>> ---
>>>>  tools/perf/util/evlist.c | 40 ++++++++++++++++++++++++++++++++++++++++
>>>>  tools/perf/util/evlist.h |  5 +++++
>>>>  2 files changed, 45 insertions(+)
>>>>
>>>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>>>> index 78c47cbafbc2..58ea562ddbd2 100644
>>>> --- a/tools/perf/util/evlist.c
>>>> +++ b/tools/perf/util/evlist.c
>>>> @@ -264,6 +264,46 @@ int evlist__add_dummy(struct evlist *evlist)
>>>>         return 0;
>>>>  }
>>>>
>>>> +static void evlist__add_system_wide(struct evlist *evlist, struct evsel *evsel)
>>>
>>> This would be:
>>> evlist__add_on_all_online_cpus(..)
>>>
>>>> +{
>>>> +       evsel->core.system_wide = true;
>>>> +
>>>> +       /* All CPUs */
>>>> +       perf_cpu_map__put(evsel->core.own_cpus);
>>>> +       evsel->core.own_cpus = perf_cpu_map__new(NULL);
>>>> +       perf_cpu_map__put(evsel->core.cpus);
>>>> +       evsel->core.cpus = perf_cpu_map__get(evsel->core.own_cpus);
>>>> +
>>>> +       /* No threads */
>>>> +       perf_thread_map__put(evsel->core.threads);
>>>> +       evsel->core.threads = perf_thread_map__new_dummy();
>>>> +
>>>> +       evlist__add(evlist, evsel);
>>>> +}
>>>> +
>>>> +struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide)
>>>> +{
>>>> +       struct evsel *evsel = evlist__dummy_event(evlist);
>>>> +
>>>> +       if (!evsel)
>>>> +               return NULL;
>>>> +
>>>> +       evsel->core.attr.exclude_kernel = 1;
>>>> +       evsel->core.attr.exclude_guest = 1;
>>>> +       evsel->core.attr.exclude_hv = 1;
>>>> +       evsel->core.attr.freq = 0;
>>>> +       evsel->core.attr.sample_period = 1;
>>>> +       evsel->no_aux_samples = true;
>>>> +       evsel->name = strdup("dummy:u");
>>>> +
>>>> +       if (system_wide)
>>>> +               evlist__add_system_wide(evlist, evsel);
>>>> +       else
>>>> +               evlist__add(evlist, evsel);
>>>> +
>>>> +       return evsel;
>>>> +}
>>>> +
>>>>  static int evlist__add_attrs(struct evlist *evlist, struct perf_event_attr *attrs, size_t nr_attrs)
>>>>  {
>>>>         struct evsel *evsel, *n;
>>>> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
>>>> index 4062f5aebfc1..dd1af114e033 100644
>>>> --- a/tools/perf/util/evlist.h
>>>> +++ b/tools/perf/util/evlist.h
>>>> @@ -114,6 +114,11 @@ int arch_evlist__add_default_attrs(struct evlist *evlist);
>>>>  struct evsel *arch_evlist__leader(struct list_head *list);
>>>>
>>>>  int evlist__add_dummy(struct evlist *evlist);
>>>> +struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide);
>>>> +static inline struct evsel *evlist__add_system_wide_dummy(struct evlist *evlist)
>>>> +{
>>>> +       return evlist__add_aux_dummy(evlist, true);
>>>> +}
>>>>
>>>>  int evlist__add_sb_event(struct evlist *evlist, struct perf_event_attr *attr,
>>>>                          evsel__sb_cb_t cb, void *data);
>>>> --
>>>> 2.25.1
>>>>
>>