linux-kernel - [PATCH 4/4] perf lock contention: Skip stack trace from BPF

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Date:   Wed,  7 Sep 2022 23:37:54 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Rogers <irogers@...gle.com>,
        linux-perf-users@...r.kernel.org, Song Liu <songliubraving@...com>,
        bpf@...r.kernel.org
Subject: [PATCH 4/4] perf lock contention: Skip stack trace from BPF

Currently it collects stack traces to max size then skip entries.
Because we don't have control how to skip perf callchains.  But BPF can
do it with bpf_get_stackid() with a flag.

Say we have max-stack=4 and stack-skip=2, we get these stack traces.

Before:                    After:

      ---> +---+ <--             ---> +---+ <--
     /     |   |    \           /     |   |    \
     |     +---+  usable        |     +---+    |
    max    |   |    /          max    |   |    |
   stack   +---+ <--          stack   +---+  usable
     |     | X |                |     |   |    |
     |     +---+   skip         |     +---+    |
     \     | X |                \     |   |    /
      ---> +---+                 ---> +---+ <--    <=== collection
                                      | X |
                                      +---+   skip
                                      | X |
                                      +---+

Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/util/bpf_lock_contention.c          | 7 ++++---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 3 ++-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index ef5323c78ffc..efe5b9968e77 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -93,6 +93,8 @@ int lock_contention_prepare(struct lock_contention *con)
 		bpf_map_update_elem(fd, &pid, &val, BPF_ANY);
 	}
 
+	skel->bss->stack_skip = con->stack_skip;
+
 	lock_contention_bpf__attach(skel);
 	return 0;
 }
@@ -127,7 +129,7 @@ int lock_contention_read(struct lock_contention *con)
 	while (!bpf_map_get_next_key(fd, &prev_key, &key)) {
 		struct map *kmap;
 		struct symbol *sym;
-		int idx;
+		int idx = 0;
 
 		bpf_map_lookup_elem(fd, &key, &data);
 		st = zalloc(sizeof(*st));
@@ -146,8 +148,7 @@ int lock_contention_read(struct lock_contention *con)
 
 		bpf_map_lookup_elem(stack, &key, stack_trace);
 
-		/* skip BPF + lock internal functions */
-		idx = con->stack_skip;
+		/* skip lock internal functions */
 		while (is_lock_function(machine, stack_trace[idx]) &&
 		       idx < con->max_stack - 1)
 			idx++;
diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 9e8b94eb6320..e107d71f0f1a 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -72,6 +72,7 @@ struct {
 int enabled;
 int has_cpu;
 int has_task;
+int stack_skip;
 
 /* error stat */
 unsigned long lost;
@@ -117,7 +118,7 @@ int contention_begin(u64 *ctx)
 	pelem->timestamp = bpf_ktime_get_ns();
 	pelem->lock = (__u64)ctx[0];
 	pelem->flags = (__u32)ctx[1];
-	pelem->stack_id = bpf_get_stackid(ctx, &stacks, BPF_F_FAST_STACK_CMP);
+	pelem->stack_id = bpf_get_stackid(ctx, &stacks, BPF_F_FAST_STACK_CMP | stack_skip);
 
 	if (pelem->stack_id < 0)
 		lost++;
-- 
2.37.2.789.g6183377224-goog