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@kernel.org>

tools/perf/util/bpf_lock_contention.c

diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index ef5323c..efe5b99 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++;

tools/perf/util/bpf_skel/lock_contention.bpf.c

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 9e8b94eb..e107d71 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++;