linux-kernel - [PATCH 3/3] perf lock contention: Do not fail EEXIST for update

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Message-ID: <20240830065150.1758962-3-namhyung@kernel.org>
Date: Thu, 29 Aug 2024 23:51:50 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.com>
Cc: Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org,
	Song Liu <song@...nel.org>,
	bpf@...r.kernel.org
Subject: [PATCH 3/3] perf lock contention: Do not fail EEXIST for update

When it updates the lock stat for the first time, it needs to create an
element in the BPF hash map.  But if there's a concurrent thread waiting
for the same lock (like for rwsem or rwlock), it might race with the
thread and possibly failed to update with -EEXIST.  In that case, it can
lookup the map again and put the data there instead of failing.

Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 4b7237e178bd..52a876b42699 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -491,6 +491,12 @@ int contention_end(u64 *ctx)
 
 		err = bpf_map_update_elem(&lock_stat, &key, &first, BPF_NOEXIST);
 		if (err < 0) {
+			if (err == -EEXIST) {
+				/* it lost the race, try to get it again */
+				data = bpf_map_lookup_elem(&lock_stat, &key);
+				if (data != NULL)
+					goto found;
+			}
 			if (err == -E2BIG)
 				data_map_full = 1;
 			__sync_fetch_and_add(&data_fail, 1);
@@ -498,6 +504,7 @@ int contention_end(u64 *ctx)
 		goto out;
 	}
 
+found:
 	__sync_fetch_and_add(&data->total_time, duration);
 	__sync_fetch_and_add(&data->count, 1);
 
-- 
2.46.0.469.g59c65b2a67-goog