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

From: Namhyung Kim
Date: Mon Sep 12 2022 - 01:53:47 EST


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@xxxxxxxxxx>
---
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