[PATCH 1/4] perf topdown: Correct leader selection with sample_read enabled

Dapeng Mi
2024-07-02T06:39:43Z

Addresses an issue where, in the absence of a topdown metrics event
within a sampling group, the slots event was incorrectly bypassed as
the sampling leader when sample_read was enabled.

perf record -e '{slots,branches}:S' -c 10000 -vv sleep 1

In this case, the slots event should be sampled as leader but the
branches event is sampled in fact like the verbose output shows.

perf_event_attr:
type 4 (cpu)
size 168
config 0x400 (slots)
sample_type IP|TID|TIME|READ|CPU|IDENTIFIER
read_format ID|GROUP|LOST
disabled 1
sample_id_all 1
exclude_guest 1
------------------------------------------------------------
sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8 = 5
------------------------------------------------------------
perf_event_attr:
type 0 (PERF_TYPE_HARDWARE)
size 168
config 0x4 (PERF_COUNT_HW_BRANCH_INSTRUCTIONS)
{ sample_period, sample_freq } 10000
sample_type IP|TID|TIME|READ|CPU|IDENTIFIER
read_format ID|GROUP|LOST
sample_id_all 1
exclude_guest 1

The sample period of slots event instead of branches event is reset to
0.

This fix ensures the slots event remains the leader under these
conditions.

Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
---
tools/perf/arch/x86/util/topdown.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/perf/arch/x86/util/topdown.c b/tools/perf/arch/x86/util/topdown.c
index 3f9a267d4501..5d7b78eb7516 100644
--- a/tools/perf/arch/x86/util/topdown.c
+++ b/tools/perf/arch/x86/util/topdown.c
@@ -1,6 +1,7 @@
// SPDX-License-Identifier: GPL-2.0
#include "api/fs/fs.h"
#include "util/evsel.h"
+#include "util/evlist.h"
#include "util/pmu.h"
#include "util/pmus.h"
#include "util/topdown.h"
@@ -41,11 +42,22 @@ bool topdown_sys_has_perf_metrics(void)
*/
bool arch_topdown_sample_read(struct evsel *leader)
{
+ struct evsel *event;
+
if (!evsel__sys_has_perf_metrics(leader))
return false;

- if (leader->core.attr.config == TOPDOWN_SLOTS)
- return true;
+ if (leader->core.attr.config != TOPDOWN_SLOTS)
+ return false;
+
+ /*
+ * If slots event as leader event but no topdown metric events in group,
+ * slots event should still sample as leader.
+ */
+ evlist__for_each_entry(leader->evlist, event) {
+ if (event != leader && strcasestr(event->name, "topdown"))
+ return true;
+ }

return false;
}
--
2.40.1