[PATCH] perf lock contention: Clear lock addr after use

[Namhyung Kim <namhyung@kernel.org>]

It checks the current lock to calculated the delta of contention time.
The address is saved in the tstamp map which is allocated at begining of
contention and released at end of contention.

But it's possible for bpf_map_delete_elem() to fail.  In that case, the
element in the tstamp map kept for the current lock and it makes the
next contention for the same lock tracked incorrectly.  Specificially
the next contention begin will see the existing element for the task and
it'd just return.  Then the next contention end will see the element and
calculate the time using the timestamp for the previous begin.

This can result in a large value for two small contentions happened from
time to time.  Let's clear the lock address so that it can be updated
next time even if the bpf_map_delete_elem() failed.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 4900a5dfb4a4..b11179452e19 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -389,6 +389,7 @@ int contention_end(u64 *ctx)
 
 	duration = bpf_ktime_get_ns() - pelem->timestamp;
 	if ((__s64)duration < 0) {
+		pelem->lock = 0;
 		bpf_map_delete_elem(&tstamp, &pid);
 		__sync_fetch_and_add(&time_fail, 1);
 		return 0;
@@ -422,6 +423,7 @@ int contention_end(u64 *ctx)
 	data = bpf_map_lookup_elem(&lock_stat, &key);
 	if (!data) {
 		if (data_map_full) {
+			pelem->lock = 0;
 			bpf_map_delete_elem(&tstamp, &pid);
 			__sync_fetch_and_add(&data_fail, 1);
 			return 0;
@@ -445,6 +447,7 @@ int contention_end(u64 *ctx)
 				data_map_full = 1;
 			__sync_fetch_and_add(&data_fail, 1);
 		}
+		pelem->lock = 0;
 		bpf_map_delete_elem(&tstamp, &pid);
 		return 0;
 	}
@@ -458,6 +461,7 @@ int contention_end(u64 *ctx)
 	if (data->min_time > duration)
 		data->min_time = duration;
 
+	pelem->lock = 0;
 	bpf_map_delete_elem(&tstamp, &pid);
 	return 0;
 }
-- 
2.42.0.582.g8ccd20d70d-goog