Re: [PATCH v2 1/4] perf lock: Add bpf maps for owner stack tracing

Namhyung Kim
2025-01-14T03:05:36Z

Hello,

On Sun, Jan 12, 2025 at 09:20:14PM -0800, Chun-Tse Shao wrote:
> Add few bpf maps in order to tracing owner stack.

If you want to split this code as a separate commit, I think you'd
better explain what these maps do and why you need them.

>
> Signed-off-by: Chun-Tse Shao <ctshao@google.com>
> ---
> tools/perf/util/bpf_lock_contention.c | 17 ++++++--
> .../perf/util/bpf_skel/lock_contention.bpf.c | 40 +++++++++++++++++--
> tools/perf/util/bpf_skel/lock_data.h | 6 +++
> 3 files changed, 56 insertions(+), 7 deletions(-)
>
> diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> index 41a1ad087895..c9c58f243ceb 100644
> --- a/tools/perf/util/bpf_lock_contention.c
> +++ b/tools/perf/util/bpf_lock_contention.c
> @@ -41,9 +41,20 @@ int lock_contention_prepare(struct lock_contention *con)
> else
> bpf_map__set_max_entries(skel->maps.task_data, 1);
>
> - if (con->save_callstack)
> - bpf_map__set_max_entries(skel->maps.stacks, con->map_nr_entries);
> - else
> + if (con->save_callstack) {
> + bpf_map__set_max_entries(skel->maps.stacks,
> + con->map_nr_entries);
> + if (con->owner) {
> + bpf_map__set_value_size(skel->maps.owner_stacks_entries,
> + con->max_stack * sizeof(u64));
> + bpf_map__set_value_size(
> + skel->maps.contention_owner_stacks,
> + con->max_stack * sizeof(u64));
> + bpf_map__set_key_size(skel->maps.owner_lock_stat,
> + con->max_stack * sizeof(u64));
> + skel->rodata->max_stack = con->max_stack;
> + }
> + } else
> bpf_map__set_max_entries(skel->maps.stacks, 1);
>
> if (target__has_cpu(target)) {
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 1069bda5d733..05da19fdab23 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -19,13 +19,37 @@
> #define LCB_F_PERCPU (1U << 4)
> #define LCB_F_MUTEX (1U << 5)
>

Can we rename these shorter and save some typings?

> -/* callstack storage */
> + /* tmp buffer for owner callstack */
> struct {
> - __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> __uint(key_size, sizeof(__u32));
> __uint(value_size, sizeof(__u64));
> + __uint(max_entries, 1);
> +} owner_stacks_entries SEC(".maps");

I think this can be 'stack_buf'.

> +
> +/* a map for tracing lock address to owner data */
> +struct {
> + __uint(type, BPF_MAP_TYPE_HASH);
> + __uint(key_size, sizeof(__u64)); // lock address
> + __uint(value_size, sizeof(cotd));
> __uint(max_entries, MAX_ENTRIES);
> -} stacks SEC(".maps");
> +} contention_owner_tracing SEC(".maps");

owner_data.

> +
> +/* a map for tracing lock address to owner stacktrace */
> +struct {
> + __uint(type, BPF_MAP_TYPE_HASH);
> + __uint(key_size, sizeof(__u64)); // lock address
> + __uint(value_size, sizeof(__u64)); // straktrace

Typo.

> + __uint(max_entries, MAX_ENTRIES);
> +} contention_owner_stacks SEC(".maps");

owner_stack.

> +
> +/* owner callstack to contention data storage */
> +struct {
> + __uint(type, BPF_MAP_TYPE_HASH);
> + __uint(key_size, sizeof(__u64));
> + __uint(value_size, sizeof(struct contention_data));
> + __uint(max_entries, MAX_ENTRIES);
> +} owner_lock_stat SEC(".maps");

owner_stat. What do you think?

By the way, I got an idea to implement stackid map in BPF using hash
map. For owner stack, you can use the stacktrace as a key and make a
value an unique integer. Then the return value can be used as a stack
id (like from bpf_get_stackid) for the owner_data and owner_stat.

Something like:

s32 get_stack_id(struct owner_stack *owner_stack, u64 stacktrace[])
{
s32 *id, new_id;
static s32 id_gen = 1;

id = bpf_map_lookup_elem(owner_stack, stacktrace);
if (id)
return *id;

new_id = __sync_fetch_and_add(&id_gen, 1);
bpf_map_update_elem(owner_stack, stacktrace, &new_id, BPF_NOEXIST);

id = bpf_map_lookup_elem(owner_stack, stacktrace);
if (id)
return *id;

return -1;
}

Later, in user space, you can traverse the owner_stack map to build
reverse mapping from id to stacktrace.

>
> /* maintain timestamp at the beginning of contention */
> struct {
> @@ -43,6 +67,14 @@ struct {
> __uint(max_entries, 1);
> } tstamp_cpu SEC(".maps");
>
> +/* callstack storage */
> +struct {
> + __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> + __uint(key_size, sizeof(__u32));
> + __uint(value_size, sizeof(__u64));
> + __uint(max_entries, MAX_ENTRIES);
> +} stacks SEC(".maps");
> +
> /* actual lock contention statistics */
> struct {
> __uint(type, BPF_MAP_TYPE_HASH);
> @@ -126,6 +158,7 @@ const volatile int needs_callstack;
> const volatile int stack_skip;
> const volatile int lock_owner;
> const volatile int use_cgroup_v2;
> +const volatile int max_stack;
>
> /* determine the key of lock stat */
> const volatile int aggr_mode;
> @@ -436,7 +469,6 @@ int contention_end(u64 *ctx)
> return 0;
> need_delete = true;
> }
> -
> duration = bpf_ktime_get_ns() - pelem->timestamp;
> if ((__s64)duration < 0) {
> __sync_fetch_and_add(&time_fail, 1);
> diff --git a/tools/perf/util/bpf_skel/lock_data.h b/tools/perf/util/bpf_skel/lock_data.h
> index de12892f992f..1ef0bca9860e 100644
> --- a/tools/perf/util/bpf_skel/lock_data.h
> +++ b/tools/perf/util/bpf_skel/lock_data.h
> @@ -3,6 +3,12 @@
> #ifndef UTIL_BPF_SKEL_LOCK_DATA_H
> #define UTIL_BPF_SKEL_LOCK_DATA_H
>
> +typedef struct contention_owner_tracing_data {
> + u32 pid; // Who has the lock.
> + u64 timestamp; // The time while the owner acquires lock and contention is going on.
> + u32 count; // How many waiters for this lock.

Switching the order of timestamp and count would remove padding.

> +} cotd;

Usually we don't use typedef to remove the struct tag.

Thanks,
Namhyung

> +
> struct tstamp_data {
> u64 timestamp;
> u64 lock;
> --
> 2.47.1.688.g23fc6f90ad-goog
>