linux-kernel - [PATCH 2/2] perf lock contention: Rework offset calculation with BPF CO-RE

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Message-ID: <20230427234833.1576130-2-namhyung@kernel.org>
Date:   Thu, 27 Apr 2023 16:48:33 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     Ian Rogers <irogers@...gle.com>,
        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, bpf@...r.kernel.org,
        Andrii Nakryiko <andrii@...nel.org>,
        Hao Luo <haoluo@...gle.com>, Song Liu <song@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: [PATCH 2/2] perf lock contention: Rework offset calculation with BPF CO-RE

It seems BPF CO-RE reloc doesn't work well with the pattern that gets
the field-offset only.  Use offsetof() to make it explicit so that
the compiler would generate the correct code.

Fixes: 0c1228486bef ("perf lock contention: Support pre-5.14 kernels")
Co-developed-by: Andrii Nakryiko <andrii.nakryiko@...il.com>
Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 30c193078bdb..8d3cfbb3cc65 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -429,21 +429,21 @@ struct rq___new {
 SEC("raw_tp/bpf_test_finish")
 int BPF_PROG(collect_lock_syms)
 {
-	__u64 lock_addr;
+	__u64 lock_addr, lock_off;
 	__u32 lock_flag;
 
+	if (bpf_core_field_exists(struct rq___new, __lock))
+		lock_off = offsetof(struct rq___new, __lock);
+	else
+		lock_off = offsetof(struct rq___old, lock);
+
 	for (int i = 0; i < MAX_CPUS; i++) {
 		struct rq *rq = bpf_per_cpu_ptr(&runqueues, i);
-		struct rq___new *rq_new = (void *)rq;
-		struct rq___old *rq_old = (void *)rq;
 
 		if (rq == NULL)
 			break;
 
-		if (bpf_core_field_exists(rq_new->__lock))
-			lock_addr = (__u64)&rq_new->__lock;
-		else
-			lock_addr = (__u64)&rq_old->lock;
+		lock_addr = (__u64)(void *)rq + lock_off;
 		lock_flag = LOCK_CLASS_RQLOCK;
 		bpf_map_update_elem(&lock_syms, &lock_addr, &lock_flag, BPF_ANY);
 	}
-- 
2.40.1.495.gc816e09b53d-goog