perf hist: Honor symbol_conf.skip_empty

So that it can skip events with no sample according to the config value.
This can omit the dummy event in the output of perf report --group.

An example output:

$ sudo perf mem record -a sleep 1
$ sudo perf report --group

Before)
#
# Samples: 232 of events 'cpu/mem-loads,ldlat=30/P, cpu/mem-stores/P, dummy:u'
# Event count (approx.): 3089861
#
# Overhead Command Shared Object Symbol
# ........................ ........... ................. .....................................
#
9.29% 0.00% 0.00% swapper [kernel.kallsyms] [k] update_blocked_averages
5.26% 0.15% 0.00% swapper [kernel.kallsyms] [k] __update_load_avg_se
4.15% 0.00% 0.00% perf-exec [kernel.kallsyms] [k] slab_update_freelist.isra.0
3.87% 0.00% 0.00% perf-exec [kernel.kallsyms] [k] memcg_slab_post_alloc_hook
3.79% 0.17% 0.00% swapper [kernel.kallsyms] [k] enqueue_task_fair
3.63% 0.00% 0.00% sleep [kernel.kallsyms] [k] next_uptodate_page
2.86% 0.00% 0.00% swapper [kernel.kallsyms] [k] __update_load_avg_cfs_rq
2.78% 0.00% 0.00% swapper [kernel.kallsyms] [k] __schedule
2.34% 0.00% 0.00% swapper [kernel.kallsyms] [k] intel_idle
2.32% 0.97% 0.00% swapper [kernel.kallsyms] [k] psi_group_change

After)
#
# Samples: 232 of events 'cpu/mem-loads,ldlat=30/P, cpu/mem-stores/P'
# Event count (approx.): 3089861
#
# Overhead Command Shared Object Symbol
# ................ ........... ................. .....................................
#
9.29% 0.00% swapper [kernel.kallsyms] [k] update_blocked_averages
5.26% 0.15% swapper [kernel.kallsyms] [k] __update_load_avg_se
4.15% 0.00% perf-exec [kernel.kallsyms] [k] slab_update_freelist.isra.0
3.87% 0.00% perf-exec [kernel.kallsyms] [k] memcg_slab_post_alloc_hook
3.79% 0.17% swapper [kernel.kallsyms] [k] enqueue_task_fair
3.63% 0.00% sleep [kernel.kallsyms] [k] next_uptodate_page
2.86% 0.00% swapper [kernel.kallsyms] [k] __update_load_avg_cfs_rq
2.78% 0.00% swapper [kernel.kallsyms] [k] __schedule
2.34% 0.00% swapper [kernel.kallsyms] [k] intel_idle
2.32% 0.97% swapper [kernel.kallsyms] [k] psi_group_change

Now it doesn't have a column for the dummy event.

Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/perf/ui/hist.c

···
 	}
 
 	for (i = 0; i < nr_members; i++) {
+		if (symbol_conf.skip_empty &&
+		    values[i].hists->stats.nr_samples == 0)
+			continue;
+
 		ret += __hpp__fmt_print(hpp, values[i].hists, values[i].val,
 					values[i].samples, fmt, len,
 					print_fn, fmtype);
···
 	int len = fmt->user_len ?: fmt->len;
 	struct evsel *evsel = hists_to_evsel(hists);
 
-	if (symbol_conf.event_group)
-		len = max(len, evsel->core.nr_members * fmt->len);
+	if (symbol_conf.event_group) {
+		int nr = 0;
+		struct evsel *pos;
+
+		for_each_group_evsel(pos, evsel) {
+			if (!symbol_conf.skip_empty ||
+			    evsel__hists(pos)->stats.nr_samples)
+				nr++;
+		}
+
+		len = max(len, nr * fmt->len);
+	}
 
 	if (len < (int)strlen(fmt->name))
 		len = strlen(fmt->name);

tools/perf/util/evsel.c

···
 #include "../perf-sys.h"
 #include "util/parse-branch-options.h"
 #include "util/bpf-filter.h"
+#include "util/hist.h"
 #include <internal/xyarray.h>
 #include <internal/lib.h>
 #include <internal/threadmap.h>
···
 int evsel__group_desc(struct evsel *evsel, char *buf, size_t size)
 {
 	int ret = 0;
+	bool first = true;
 	struct evsel *pos;
 	const char *group_name = evsel__group_name(evsel);
 
 	if (!evsel->forced_leader)
 		ret = scnprintf(buf, size, "%s { ", group_name);
 
-	ret += scnprintf(buf + ret, size - ret, "%s", evsel__name(evsel));
+	for_each_group_evsel(pos, evsel) {
+		if (symbol_conf.skip_empty &&
+		    evsel__hists(pos)->stats.nr_samples == 0)
+			continue;
 
-	for_each_group_member(pos, evsel)
-		ret += scnprintf(buf + ret, size - ret, ", %s", evsel__name(pos));
+		ret += scnprintf(buf + ret, size - ret, "%s%s",
+				 first ? "" : ", ", evsel__name(pos));
+		first = false;
+	}
 
 	if (!evsel->forced_leader)
 		ret += scnprintf(buf + ret, size - ret, " }");

tools/perf/util/python.c

···
 #include "util/env.h"
 #include "util/pmu.h"
 #include "util/pmus.h"
+#include "util/symbol_conf.h"
 #include <internal/lib.h>
 #include "util.h"
 
···
 #ifndef Py_TYPE
 #define Py_TYPE(ob) (((PyObject*)(ob))->ob_type)
 #endif
+
+struct symbol_conf symbol_conf;
 
 /*
  * Avoid bringing in event parsing.