[PATCH v5 2/5] perf lock: Retrieve owner callstack in bpf program

Chun-Tse Shao
2025-02-12T22:30:13Z

This implements per-callstack aggregation of lock owners in addition to
per-thread. The owner callstack is captured using `bpf_get_task_stack()`
at `contention_begin()` and it also adds a custom stackid function for the
owner stacks to be compared easily.

The owner info is kept in a hash map using lock addr as a key to handle
multiple waiters for the same lock. At `contention_end()`, it updates the
owner lock stat based on the info that was saved at `contention_begin()`.
If there are more waiters, it'd update the owner pid to itself as
`contention_end()` means it gets the lock now. But it also needs to check
the return value of the lock function in case task was killed by a signal
or something.

Signed-off-by: Chun-Tse Shao <ctshao@google.com>
---
.../perf/util/bpf_skel/lock_contention.bpf.c | 218 +++++++++++++++++-
1 file changed, 209 insertions(+), 9 deletions(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 23fe9cc980ae..e8b113d5802a 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -197,6 +197,9 @@ int data_fail;
int task_map_full;
int data_map_full;

+struct task_struct *bpf_task_from_pid(s32 pid) __ksym __weak;
+void bpf_task_release(struct task_struct *p) __ksym __weak;
+
static inline __u64 get_current_cgroup_id(void)
{
struct task_struct *task;
@@ -420,6 +423,61 @@ static inline struct tstamp_data *get_tstamp_elem(__u32 flags)
return pelem;
}

+static inline s32 get_owner_stack_id(u64 *stacktrace)
+{
+ s32 *id, new_id;
+ static s64 id_gen = 1;
+
+ id = bpf_map_lookup_elem(&owner_stacks, stacktrace);
+ if (id)
+ return *id;
+
+ new_id = (s32)__sync_fetch_and_add(&id_gen, 1);
+
+ bpf_map_update_elem(&owner_stacks, stacktrace, &new_id, BPF_NOEXIST);
+
+ id = bpf_map_lookup_elem(&owner_stacks, stacktrace);
+ if (id)
+ return *id;
+
+ return -1;
+}
+
+static inline void update_contention_data(struct contention_data *data, u64 duration, u32 count)
+{
+ __sync_fetch_and_add(&data->total_time, duration);
+ __sync_fetch_and_add(&data->count, count);
+
+ /* FIXME: need atomic operations */
+ if (data->max_time < duration)
+ data->max_time = duration;
+ if (data->min_time > duration)
+ data->min_time = duration;
+}
+
+static inline void update_owner_stat(u32 id, u64 duration, u32 flags)
+{
+ struct contention_key key = {
+ .stack_id = id,
+ .pid = 0,
+ .lock_addr_or_cgroup = 0,
+ };
+ struct contention_data *data = bpf_map_lookup_elem(&owner_stat, &key);
+
+ if (!data) {
+ struct contention_data first = {
+ .total_time = duration,
+ .max_time = duration,
+ .min_time = duration,
+ .count = 1,
+ .flags = flags,
+ };
+ bpf_map_update_elem(&owner_stat, &key, &first, BPF_NOEXIST);
+ } else {
+ update_contention_data(data, duration, 1);
+ }
+}
+
SEC("tp_btf/contention_begin")
int contention_begin(u64 *ctx)
{
@@ -437,6 +495,72 @@ int contention_begin(u64 *ctx)
pelem->flags = (__u32)ctx[1];

if (needs_callstack) {
+ u32 i = 0;
+ u32 id = 0;
+ int owner_pid;
+ u64 *buf;
+ struct task_struct *task;
+ struct owner_tracing_data *otdata;
+
+ if (!lock_owner)
+ goto skip_owner;
+
+ task = get_lock_owner(pelem->lock, pelem->flags);
+ if (!task)
+ goto skip_owner;
+
+ owner_pid = BPF_CORE_READ(task, pid);
+
+ buf = bpf_map_lookup_elem(&stack_buf, &i);
+ if (!buf)
+ goto skip_owner;
+ for (i = 0; i < max_stack; i++)
+ buf[i] = 0x0;
+
+ if (!bpf_task_from_pid)
+ goto skip_owner;
+
+ task = bpf_task_from_pid(owner_pid);
+ if (!task)
+ goto skip_owner;
+
+ bpf_get_task_stack(task, buf, max_stack * sizeof(unsigned long), 0);
+ bpf_task_release(task);
+
+ otdata = bpf_map_lookup_elem(&owner_data, &pelem->lock);
+ id = get_owner_stack_id(buf);
+
+ /*
+ * Contention just happens, or corner case `lock` is owned by process not
+ * `owner_pid`. For the corner case we treat it as unexpected internal error and
+ * just ignore the precvious tracing record.
+ */
+ if (!otdata || otdata->pid != owner_pid) {
+ struct owner_tracing_data first = {
+ .pid = owner_pid,
+ .timestamp = pelem->timestamp,
+ .count = 1,
+ .stack_id = id,
+ };
+ bpf_map_update_elem(&owner_data, &pelem->lock, &first, BPF_ANY);
+ }
+ /* Contention is ongoing and new waiter joins */
+ else {
+ __sync_fetch_and_add(&otdata->count, 1);
+
+ /*
+ * The owner is the same, but stacktrace might be changed. In this case we
+ * store/update `owner_stat` based on current owner stack id.
+ */
+ if (id != otdata->stack_id) {
+ update_owner_stat(id, pelem->timestamp - otdata->timestamp,
+ pelem->flags);
+
+ otdata->timestamp = pelem->timestamp;
+ otdata->stack_id = id;
+ }
+ }
+skip_owner:
pelem->stack_id = bpf_get_stackid(ctx, &stacks,
BPF_F_FAST_STACK_CMP | stack_skip);
if (pelem->stack_id < 0)
@@ -473,6 +597,7 @@ int contention_end(u64 *ctx)
struct tstamp_data *pelem;
struct contention_key key = {};
struct contention_data *data;
+ __u64 timestamp;
__u64 duration;
bool need_delete = false;

@@ -500,12 +625,94 @@ int contention_end(u64 *ctx)
need_delete = true;
}

- duration = bpf_ktime_get_ns() - pelem->timestamp;
+ timestamp = bpf_ktime_get_ns();
+ duration = timestamp - pelem->timestamp;
if ((__s64)duration < 0) {
__sync_fetch_and_add(&time_fail, 1);
goto out;
}

+ if (needs_callstack && lock_owner) {
+ struct owner_tracing_data *otdata = bpf_map_lookup_elem(&owner_data, &pelem->lock);
+
+ if (!otdata)
+ goto skip_owner;
+
+ /* Update `owner_stat` */
+ update_owner_stat(otdata->stack_id, timestamp - otdata->timestamp, pelem->flags);
+
+ /* No contention is occurring, delete `lock` entry in `owner_data` */
+ if (otdata->count <= 1)
+ bpf_map_delete_elem(&owner_data, &pelem->lock);
+ /*
+ * Contention is still ongoing, with a new owner (current task). `owner_data`
+ * should be updated accordingly.
+ */
+ else {
+ u32 i = 0;
+ s32 ret = (s32)ctx[1];
+ u64 *buf;
+
+ __sync_fetch_and_add(&otdata->count, -1);
+
+ buf = bpf_map_lookup_elem(&stack_buf, &i);
+ if (!buf)
+ goto skip_owner;
+ for (i = 0; i < (u32)max_stack; i++)
+ buf[i] = 0x0;
+
+ /*
+ * `ret` has the return code of the lock function.
+ * If `ret` is negative, the current task terminates lock waiting without
+ * acquiring it. Owner is not changed, but we still need to update the owner
+ * stack.
+ */
+ if (ret < 0) {
+ s32 id = 0;
+ struct task_struct *task;
+
+ if (!bpf_task_from_pid)
+ goto skip_owner;
+
+ task = bpf_task_from_pid(otdata->pid);
+ if (!task)
+ goto skip_owner;
+
+ bpf_get_task_stack(task, buf,
+ max_stack * sizeof(unsigned long), 0);
+ bpf_task_release(task);
+
+ id = get_owner_stack_id(buf);
+
+ /*
+ * If owner stack is changed, update `owner_data` and `owner_stat`
+ * accordingly.
+ */
+ if (id != otdata->stack_id) {
+ update_owner_stat(id, pelem->timestamp - otdata->timestamp,
+ pelem->flags);
+
+ otdata->timestamp = pelem->timestamp;
+ otdata->stack_id = id;
+ }
+ }
+ /*
+ * Otherwise, update tracing data with the current task, which is the new
+ * owner.
+ */
+ else {
+ otdata->pid = pid;
+ otdata->timestamp = timestamp;
+ /*
+ * We don't want to retrieve callstack here, since it is where the
+ * current task acquires the lock and provides no additional
+ * information. We simply assign -1 to invalidate it.
+ */
+ otdata->stack_id = -1;
+ }
+ }
+ }
+skip_owner:
switch (aggr_mode) {
case LOCK_AGGR_CALLER:
key.stack_id = pelem->stack_id;
@@ -589,14 +796,7 @@ int contention_end(u64 *ctx)
}

found:
- __sync_fetch_and_add(&data->total_time, duration);
- __sync_fetch_and_add(&data->count, 1);
-
- /* FIXME: need atomic operations */
- if (data->max_time < duration)
- data->max_time = duration;
- if (data->min_time > duration)
- data->min_time = duration;
+ update_contention_data(data, duration, 1);

out:
pelem->lock = 0;