Re: [PATCH v7 4/4] perf lock: Report owner stack in usermode

[Athira Rajeev <atrajeev@linux.ibm.com>]

> On 25 Feb 2025, at 12:12 AM, Chun-Tse Shao <ctshao@google.com> wrote:
> 
> This patch parses `owner_lock_stat` into a RB tree, enabling ordered
> reporting of owner lock statistics with stack traces. It also updates
> the documentation for the `-o` option in contention mode, decouples `-o`
> from `-t`, and issues a warning to inform users about the new behavior
> of `-ov`.
> 
> Example output:
>  $ sudo ~/linux/tools/perf/perf lock con -abvo -Y mutex-spin -E3 perf bench sched pipe
>  ...
>   contended   total wait     max wait     avg wait         type   caller
> 
>         171      1.55 ms     20.26 us      9.06 us        mutex   pipe_read+0x57
>                          0xffffffffac6318e7  pipe_read+0x57
>                          0xffffffffac623862  vfs_read+0x332
>                          0xffffffffac62434b  ksys_read+0xbb
>                          0xfffffffface604b2  do_syscall_64+0x82
>                          0xffffffffad00012f  entry_SYSCALL_64_after_hwframe+0x76
>          36    193.71 us     15.27 us      5.38 us        mutex   pipe_write+0x50
>                          0xffffffffac631ee0  pipe_write+0x50
>                          0xffffffffac6241db  vfs_write+0x3bb
>                          0xffffffffac6244ab  ksys_write+0xbb
>                          0xfffffffface604b2  do_syscall_64+0x82
>                          0xffffffffad00012f  entry_SYSCALL_64_after_hwframe+0x76
>           4     51.22 us     16.47 us     12.80 us        mutex   do_epoll_wait+0x24d
>                          0xffffffffac691f0d  do_epoll_wait+0x24d
>                          0xffffffffac69249b  do_epoll_pwait.part.0+0xb
>                          0xffffffffac693ba5  __x64_sys_epoll_pwait+0x95
>                          0xfffffffface604b2  do_syscall_64+0x82
>                          0xffffffffad00012f  entry_SYSCALL_64_after_hwframe+0x76
> 
>  === owner stack trace ===
> 
>           3     31.24 us     15.27 us     10.41 us        mutex   pipe_read+0x348
>                          0xffffffffac631bd8  pipe_read+0x348
>                          0xffffffffac623862  vfs_read+0x332
>                          0xffffffffac62434b  ksys_read+0xbb
>                          0xfffffffface604b2  do_syscall_64+0x82
>                          0xffffffffad00012f  entry_SYSCALL_64_after_hwframe+0x76
>  ...
> 
> Signed-off-by: Chun-Tse Shao <ctshao@google.com>
> ---
> tools/perf/Documentation/perf-lock.txt |  5 ++-
> tools/perf/builtin-lock.c              | 22 +++++++++-
> tools/perf/util/bpf_lock_contention.c  | 57 ++++++++++++++++++++++++++
> tools/perf/util/lock-contention.h      |  7 ++++
> 4 files changed, 87 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/Documentation/perf-lock.txt b/tools/perf/Documentation/perf-lock.txt
> index d3793054f7d3..859dc11a7372 100644
> --- a/tools/perf/Documentation/perf-lock.txt
> +++ b/tools/perf/Documentation/perf-lock.txt
> @@ -179,8 +179,9 @@ CONTENTION OPTIONS
> 
> -o::
> --lock-owner::
> - Show lock contention stat by owners.  Implies --threads and
> - requires --use-bpf.
> + Show lock contention stat by owners. This option can be combined with -t,
> + which shows owner's per thread lock stats, or -v, which shows owner's
> + stacktrace. Requires --use-bpf.
> 
> -Y::
> --type-filter=<value>::
> diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
> index 9bebc186286f..05e7bc30488a 100644
> --- a/tools/perf/builtin-lock.c
> +++ b/tools/perf/builtin-lock.c
> @@ -1817,6 +1817,22 @@ static void print_contention_result(struct lock_contention *con)
> break;
> }
> 
> + if (con->owner && con->save_callstack && verbose > 0) {
> + struct rb_root root = RB_ROOT;
> +
> + if (symbol_conf.field_sep)
> + fprintf(lock_output, "# owner stack trace:\n");
> + else
> + fprintf(lock_output, "\n=== owner stack trace ===\n\n");
> + while ((st = pop_owner_stack_trace(con)))
> + insert_to(&root, st, compare);
> +
> + while ((st = pop_from(&root))) {
> + print_lock_stat(con, st);
> + free(st);
> + }
> + }
> +
> if (print_nr_entries) {
> /* update the total/bad stats */
> while ((st = pop_from_result())) {
> @@ -1962,8 +1978,10 @@ static int check_lock_contention_options(const struct option *options,
> }
> }
> 
> - if (show_lock_owner)
> - show_thread_stats = true;
> + if (show_lock_owner && !show_thread_stats) {
> + pr_warning("Now -o try to show owner's callstack instead of pid and comm.\n");
> + pr_warning("Please use -t option too to keep the old behavior.\n");
> + }
> 
> return 0;
> }
> diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> index 76542b86e83f..16f4deba69ec 100644
> --- a/tools/perf/util/bpf_lock_contention.c
> +++ b/tools/perf/util/bpf_lock_contention.c
> @@ -549,6 +549,63 @@ static const char *lock_contention_get_name(struct lock_contention *con,
> return name_buf;
> }
> 
> +struct lock_stat *pop_owner_stack_trace(struct lock_contention *con)
> +{
> + int stacks_fd, stat_fd;
> + u64 *stack_trace = NULL;
> + s32 stack_id;
> + struct contention_key ckey = {};
> + struct contention_data cdata = {};
> + size_t stack_size = con->max_stack * sizeof(*stack_trace);
> + struct lock_stat *st = NULL;
> +
> + stacks_fd = bpf_map__fd(skel->maps.owner_stacks);
> + stat_fd = bpf_map__fd(skel->maps.owner_stat);
> + if (!stacks_fd || !stat_fd)
> + goto out_err;
> +
> + stack_trace = zalloc(stack_size);
> + if (stack_trace == NULL)
> + goto out_err;
> +
> + if (bpf_map_get_next_key(stacks_fd, NULL, stack_trace))
> + goto out_err;
> +
> + bpf_map_lookup_elem(stacks_fd, stack_trace, &stack_id);
> + ckey.stack_id = stack_id;
> + bpf_map_lookup_elem(stat_fd, &ckey, &cdata);
> +
> + st = zalloc(sizeof(struct lock_stat));
> + if (!st)
> + goto out_err;
> +
> + st->name = strdup(stack_trace[0] ? lock_contention_get_name(con, NULL, stack_trace, 0) :
> +   "unknown");

Hi,

I am hitting a compilation issue with this change. Sorry for responding late. I tried with change from tmp.perf-tools-next and hit below issue:


  CC      util/bpf_lock_contention.o
util/bpf_lock_contention.c: In function ‘lock_contention_get_name’:
cc1: error: function may return address of local variable [-Werror=return-local-addr]
util/bpf_lock_contention.c:470:45: note: declared here
  470 |                 struct contention_task_data task;
      |                                             ^~~~
cc1: all warnings being treated as errors
make[4]: *** [/root/perf-tools-next/tools/build/Makefile.build:85: util/bpf_lock_contention.o] Error 1
make[4]: *** Waiting for unfinished jobs....
  LD      perf-in.o
make[3]: *** [/root/perf-tools-next/tools/build/Makefile.build:138: util] Error 2
make[2]: *** [Makefile.perf:822: perf-util-in.o] Error 2
make[1]: *** [Makefile.perf:321: sub-make] Error 2
make: *** [Makefile:76: all] Error 2


Code snippet:

if (con->aggr_mode == LOCK_AGGR_TASK) {
                struct contention_task_data task;
                int pid = key->pid;
                int task_fd = bpf_map__fd(skel->maps.task_data);

                /* do not update idle comm which contains CPU number */
                if (pid) {
                        struct thread *t = machine__findnew_thread(machine, /*pid=*/-1, pid);

                        if (t == NULL)
                                return name;
                        if (!bpf_map_lookup_elem(task_fd, &pid, &task) &&
                            thread__set_comm(t, task.comm, /*timestamp=*/0))
                                name = task.comm;
                }
                return name;
        }


We are calling lock_contention_get_name with second argument as NULL .
Though error above points to “contention_task_data”, I think the local variable here is for “name” ?


Thanks
Athira

> + if (!st->name)
> + goto out_err;
> +
> + st->flags = cdata.flags;
> + st->nr_contended = cdata.count;
> + st->wait_time_total = cdata.total_time;
> + st->wait_time_max = cdata.max_time;
> + st->wait_time_min = cdata.min_time;
> + st->callstack = stack_trace;
> +
> + if (cdata.count)
> + st->avg_wait_time = cdata.total_time / cdata.count;
> +
> + bpf_map_delete_elem(stacks_fd, stack_trace);
> + bpf_map_delete_elem(stat_fd, &ckey);
> +
> + return st;
> +
> +out_err:
> + free(stack_trace);
> + free(st);
> +
> + return NULL;
> +}
> +
> int lock_contention_read(struct lock_contention *con)
> {
> int fd, stack, err = 0;
> diff --git a/tools/perf/util/lock-contention.h b/tools/perf/util/lock-contention.h
> index a09f7fe877df..97fd33c57f17 100644
> --- a/tools/perf/util/lock-contention.h
> +++ b/tools/perf/util/lock-contention.h
> @@ -168,6 +168,8 @@ int lock_contention_stop(void);
> int lock_contention_read(struct lock_contention *con);
> int lock_contention_finish(struct lock_contention *con);
> 
> +struct lock_stat *pop_owner_stack_trace(struct lock_contention *con);
> +
> #else  /* !HAVE_BPF_SKEL */
> 
> static inline int lock_contention_prepare(struct lock_contention *con __maybe_unused)
> @@ -187,6 +189,11 @@ static inline int lock_contention_read(struct lock_contention *con __maybe_unuse
> return 0;
> }
> 
> +struct lock_stat *pop_owner_stack_trace(struct lock_contention *con)
> +{
> + return NULL;
> +}
> +
> #endif  /* HAVE_BPF_SKEL */
> 
> #endif  /* PERF_LOCK_CONTENTION_H */
> -- 
> 2.48.1.658.g4767266eb4-goog
> 
>