[PATCH v2 2/4] perf lock: Retrieve owner callstack in bpf program

Chun-Tse Shao
2025-01-13T05:23:10Z

Tracing owner callstack in `contention_begin()` and `contention_end()`,
and storing in `owner_lock_stat` bpf map.

Signed-off-by: Chun-Tse Shao <ctshao@google.com>
---
.../perf/util/bpf_skel/lock_contention.bpf.c | 152 +++++++++++++++++-
1 file changed, 151 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 05da19fdab23..3f47fbfa237c 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -7,6 +7,7 @@
#include <asm-generic/errno-base.h>

#include "lock_data.h"
+#include <time.h>

/* for collect_lock_syms(). 4096 was rejected by the verifier */
#define MAX_CPUS 1024
@@ -178,6 +179,9 @@ int data_fail;
int task_map_full;
int data_map_full;

+struct task_struct *bpf_task_from_pid(s32 pid) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
static inline __u64 get_current_cgroup_id(void)
{
struct task_struct *task;
@@ -407,6 +411,60 @@ int contention_begin(u64 *ctx)
pelem->flags = (__u32)ctx[1];

if (needs_callstack) {
+ u32 i = 0;
+ int owner_pid;
+ unsigned long *entries;
+ struct task_struct *task;
+ cotd *data;
+
+ if (!lock_owner)
+ goto contention_begin_skip_owner_callstack;
+
+ task = get_lock_owner(pelem->lock, pelem->flags);
+ if (!task)
+ goto contention_begin_skip_owner_callstack;
+
+ owner_pid = BPF_CORE_READ(task, pid);
+
+ entries = bpf_map_lookup_elem(&owner_stacks_entries, &i);
+ if (!entries)
+ goto contention_begin_skip_owner_callstack;
+ for (i = 0; i < max_stack; i++)
+ entries[i] = 0x0;
+
+ task = bpf_task_from_pid(owner_pid);
+ if (task) {
+ bpf_get_task_stack(task, entries,
+ max_stack * sizeof(unsigned long),
+ 0);
+ bpf_task_release(task);
+ }
+
+ data = bpf_map_lookup_elem(&contention_owner_tracing,
+ &(pelem->lock));
+
+ // Contention just happens, or corner case `lock` is owned by
+ // process not `owner_pid`.
+ if (!data || data->pid != owner_pid) {
+ cotd first = {
+ .pid = owner_pid,
+ .timestamp = pelem->timestamp,
+ .count = 1,
+ };
+ bpf_map_update_elem(&contention_owner_tracing,
+ &(pelem->lock), &first, BPF_ANY);
+ bpf_map_update_elem(&contention_owner_stacks,
+ &(pelem->lock), entries, BPF_ANY);
+ }
+ // Contention is going on and new waiter joins.
+ else {
+ __sync_fetch_and_add(&data->count, 1);
+ // TODO: Since for owner the callstack would change at
+ // different time, We should check and report if the
+ // callstack is different with the recorded one in
+ // `contention_owner_stacks`.
+ }
+contention_begin_skip_owner_callstack:
pelem->stack_id = bpf_get_stackid(ctx, &stacks,
BPF_F_FAST_STACK_CMP | stack_skip);
if (pelem->stack_id < 0)
@@ -443,6 +501,7 @@ 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;

@@ -469,12 +528,103 @@ int contention_end(u64 *ctx)
return 0;
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) {
+ u64 owner_contention_time;
+ unsigned long *owner_stack;
+ struct contention_data *cdata;
+ cotd *otdata;
+
+ otdata = bpf_map_lookup_elem(&contention_owner_tracing,
+ &(pelem->lock));
+ owner_stack = bpf_map_lookup_elem(&contention_owner_stacks,
+ &(pelem->lock));
+ if (!otdata || !owner_stack)
+ goto contention_end_skip_owner_callstack;
+
+ owner_contention_time = timestamp - otdata->timestamp;
+
+ // Update `owner_lock_stat` if `owner_stack` is
+ // available.
+ if (owner_stack[0] != 0x0) {
+ cdata = bpf_map_lookup_elem(&owner_lock_stat,
+ owner_stack);
+ if (!cdata) {
+ struct contention_data first = {
+ .total_time = owner_contention_time,
+ .max_time = owner_contention_time,
+ .min_time = owner_contention_time,
+ .count = 1,
+ .flags = pelem->flags,
+ };
+ bpf_map_update_elem(&owner_lock_stat,
+ owner_stack, &first,
+ BPF_ANY);
+ } else {
+ __sync_fetch_and_add(&cdata->total_time,
+ owner_contention_time);
+ __sync_fetch_and_add(&cdata->count, 1);
+
+ /* FIXME: need atomic operations */
+ if (cdata->max_time < owner_contention_time)
+ cdata->max_time = owner_contention_time;
+ if (cdata->min_time > owner_contention_time)
+ cdata->min_time = owner_contention_time;
+ }
+ }
+
+ // No contention is going on, delete `lock` in
+ // `contention_owner_tracing` and
+ // `contention_owner_stacks`
+ if (otdata->count <= 1) {
+ bpf_map_delete_elem(&contention_owner_tracing,
+ &(pelem->lock));
+ bpf_map_delete_elem(&contention_owner_stacks,
+ &(pelem->lock));
+ }
+ // Contention is still going on, with a new owner
+ // (current task). `otdata` should be updated accordingly.
+ else {
+ (otdata->count)--;
+
+ // If ctx[1] is not 0, the current task terminates lock
+ // waiting without acquiring it. Owner is not changed.
+ if (ctx[1] == 0) {
+ u32 i = 0;
+ unsigned long *entries = bpf_map_lookup_elem(
+ &owner_stacks_entries, &i);
+ if (entries) {
+ for (i = 0; i < (u32)max_stack; i++)
+ entries[i] = 0x0;
+
+ bpf_get_task_stack(
+ bpf_get_current_task_btf(),
+ entries,
+ max_stack *
+ sizeof(unsigned long),
+ 0);
+ bpf_map_update_elem(
+ &contention_owner_stacks,
+ &(pelem->lock), entries,
+ BPF_ANY);
+ }
+
+ otdata->pid = pid;
+ otdata->timestamp = timestamp;
+ }
+
+ bpf_map_update_elem(&contention_owner_tracing,
+ &(pelem->lock), otdata, BPF_ANY);
+ }
+ }
+contention_end_skip_owner_callstack:
+
switch (aggr_mode) {
case LOCK_AGGR_CALLER:
key.stack_id = pelem->stack_id;
--
2.47.1.688.g23fc6f90ad-goog