Re: [PATCH V2 13/23] perf evlist: Add evlist__add_dummy_on_all_cpus()

[Ian Rogers <irogers@google.com>]

On Fri, May 6, 2022 at 5:26 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> Add evlist__add_dummy_on_all_cpus() to enable creating a system-wide dummy
> event that sets up the system-wide maps before map propagation.
>
> 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@intel.com>
> ---
>  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..c16bd4836314 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_on_all_cpus(struct evlist *evlist, struct evsel *evsel)
> +{
> +       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_on_all_cpus(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..1bde9ccf4e7d 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_dummy_on_all_cpus(struct evlist *evlist)

Sorry to be a language lawyer. What I hope to clean up with CPU maps is that:

empty == dummy == any CPU
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/cpumap.c?h=perf/core#n279

Given every CPU map should be empty or contain any CPU then it seems
they all meet the definition of empty - so something is wrong.

The cpu map here is explicitly opened so that it gets all online CPUs:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/cpumap.c?h=perf/core#n174

From:
https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/cputopology.rst
there are example topologies like:
kernel_max: 31
   offline: 2,4-31,32-63
    online: 0-1,3
  possible: 0-31
   present: 0-31

all_cpus could mean the union of offline and online CPUs, possible
CPUs or present CPUs. You are saying that in the perf code all_cpus
should be the same as all online cpus as only those CPUs are valid
with perf_event_open. That's true but offline CPUs can be made online.
If that happens here then the dummy events will have a CPU map that
rather than being for all CPUs will be for all online CPUs at the
point it was opened. Having online in the function name I think
captures the time dependent nature of this - but if you think that's
too much could we add a comment? I'm trying to avoid a situation, like
with the CPU map code, where all and online are interchangeable
leading to the code being unnecessarily confusing unless you read
every line.

Thanks,
Ian

> +{
> +       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
>