[PATCH] perf evsel: Get group fd from CPU0 for system wide event

[Jin Yao]

A metric may consist of system wide event and non system-wide event.
The event group leader may be the system wide event.

For example, the metric "C2_Pkg_Residency" consists of
"cstate_pkg/c2-residency" and "msr/tsc". The former counts on the first
CPU of socket (tagged system-wide) and the latter is per CPU.

But "C2_Pkg_Residency" hits assertion failure on cascadelakex.

 # perf stat -M "C2_Pkg_Residency" -a -- sleep 1
 perf: util/evsel.c:1464: get_group_fd: Assertion `!(fd == -1)' failed.
 Aborted

get_group_fd(evsel, cpu, thread)
{
	leader = evsel->leader;
	fd = FD(leader, cpu, thread);
	BUG_ON(fd == -1);
}

Considering this case, leader is "cstate_pkg/c2-residency", evsel is
"msr/tsc" and cpu is 1. Because "cstate_pkg/c2-residency" is a system-wide
event and it's processed on CPU0, so FD(leader, 1, thread) must return an
invalid fd, then BUG_ON() may be triggered.

This patch gets group fd from CPU0 for system wide event if
FD(leader, cpu, thread) returns invalid fd.

With this patch,

 # perf stat -M "C2_Pkg_Residency" -a -- sleep 1

 Performance counter stats for 'system wide':

        1000850802      cstate_pkg/c2-residency/  #      0.5 C2_Pkg_Residency
      201446161592      msr/tsc/

       1.010637051 seconds time elapsed

Fixes: 6a4bb04caacc ("perf tools: Enable grouping logic for parsed events")
Signed-off-by: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
---
 tools/perf/util/evsel.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 6a571d322bb2..cd6470f63d6f 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1461,6 +1461,9 @@ static int get_group_fd(struct evsel *evsel, int cpu, int thread)
 	BUG_ON(!leader->core.fd);
 
 	fd = FD(leader, cpu, thread);
+	if (fd == -1 && leader->core.system_wide)
+		fd = FD(leader, 0, thread);
+
 	BUG_ON(fd == -1);
 
 	return fd;
-- 
2.17.1