Re: [PATCH v2 13/13] perf machine thread: Remove exited threads by default

From: Adrian Hunter
Date: Mon Oct 23 2023 - 10:24:15 EST

On 12/10/23 09:23, Ian Rogers wrote:
> struct thread values hold onto references to mmaps, dsos, etc. When a
> thread exits it is necessary to clean all of this memory up by
> removing the thread from the machine's threads. Some tools require
> this doesn't happen, such as perf report if offcpu events exist or if
> a task list is being generated, so add a symbol_conf value to make the
> behavior optional. When an exited thread is left in the machine's
> threads, mark it as exited.
>
> This change relates to commit 40826c45eb0b ("perf thread: Remove
> notion of dead threads"). Dead threads were removed as they had a
> reference count of 0 and were difficult to reason about with the
> reference count checker. Here a thread is removed from threads when it
> exits, unless via symbol_conf the exited thread isn't remove and is
> marked as exited. Reference counting behaves as it normally does.

Can we exclude AUX area tracing?

Essentially, the EXIT event happens when the task is still running
in kernel mode, so the thread has not in fact fully exited.

Example:

# perf record -a --kcore -e intel_pt// uname

Before:

# perf script --itrace=b --show-task-events -C6 | grep -C10 EXIT
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63124ee __perf_event_header__init_id+0x5e ([kernel.kallsyms]) => ffffffffb63124f7 __perf_event_header__init_id+0x67 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6312501 __perf_event_header__init_id+0x71 ([kernel.kallsyms]) => ffffffffb6312512 __perf_event_header__init_id+0x82 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6312531 __perf_event_header__init_id+0xa1 ([kernel.kallsyms]) => ffffffffb6316b3a perf_event_task_output+0x26a ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6316b40 perf_event_task_output+0x270 ([kernel.kallsyms]) => ffffffffb6316959 perf_event_task_output+0x89 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6316966 perf_event_task_output+0x96 ([kernel.kallsyms]) => ffffffffb6322040 perf_output_begin+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6322080 perf_output_begin+0x40 ([kernel.kallsyms]) => ffffffffb6194dc0 __rcu_read_lock+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6194de1 __rcu_read_lock+0x21 ([kernel.kallsyms]) => ffffffffb6322085 perf_output_begin+0x45 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63220ce perf_output_begin+0x8e ([kernel.kallsyms]) => ffffffffb611d280 preempt_count_add+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb611d2bf preempt_count_add+0x3f ([kernel.kallsyms]) => ffffffffb63220d3 perf_output_begin+0x93 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63220e8 perf_output_begin+0xa8 ([kernel.kallsyms]) => ffffffffb63220ff perf_output_begin+0xbf ([kernel.kallsyms])
uname 14740 [006] 26795.092638: PERF_RECORD_EXIT(14740:14740):(14739:14739)
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6322119 perf_output_begin+0xd9 ([kernel.kallsyms]) => ffffffffb6322128 perf_output_begin+0xe8 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6322146 perf_output_begin+0x106 ([kernel.kallsyms]) => ffffffffb63220ea perf_output_begin+0xaa ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63220f9 perf_output_begin+0xb9 ([kernel.kallsyms]) => ffffffffb63221ab perf_output_begin+0x16b ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63221ae perf_output_begin+0x16e ([kernel.kallsyms]) => ffffffffb63221b6 perf_output_begin+0x176 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6322202 perf_output_begin+0x1c2 ([kernel.kallsyms]) => ffffffffb6322167 perf_output_begin+0x127 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb632218c perf_output_begin+0x14c ([kernel.kallsyms]) => ffffffffb631696b perf_event_task_output+0x9b ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6316990 perf_event_task_output+0xc0 ([kernel.kallsyms]) => ffffffffb61034a0 __task_pid_nr_ns+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb61034b7 __task_pid_nr_ns+0x17 ([kernel.kallsyms]) => ffffffffb6194dc0 __rcu_read_lock+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6194de1 __rcu_read_lock+0x21 ([kernel.kallsyms]) => ffffffffb61034bc __task_pid_nr_ns+0x1c ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6103503 __task_pid_nr_ns+0x63 ([kernel.kallsyms]) => ffffffffb610353b __task_pid_nr_ns+0x9b ([kernel.kallsyms])

After:

$ perf script --itrace=b --show-task-events -C6 | grep -C10 EXIT
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63124ee __perf_event_header__init_id+0x5e ([kernel.kallsyms]) => ffffffffb63124f7 __perf_event_header__init_id+0x67 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6312501 __perf_event_header__init_id+0x71 ([kernel.kallsyms]) => ffffffffb6312512 __perf_event_header__init_id+0x82 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6312531 __perf_event_header__init_id+0xa1 ([kernel.kallsyms]) => ffffffffb6316b3a perf_event_task_output+0x26a ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6316b40 perf_event_task_output+0x270 ([kernel.kallsyms]) => ffffffffb6316959 perf_event_task_output+0x89 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6316966 perf_event_task_output+0x96 ([kernel.kallsyms]) => ffffffffb6322040 perf_output_begin+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6322080 perf_output_begin+0x40 ([kernel.kallsyms]) => ffffffffb6194dc0 __rcu_read_lock+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb6194de1 __rcu_read_lock+0x21 ([kernel.kallsyms]) => ffffffffb6322085 perf_output_begin+0x45 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63220ce perf_output_begin+0x8e ([kernel.kallsyms]) => ffffffffb611d280 preempt_count_add+0x0 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb611d2bf preempt_count_add+0x3f ([kernel.kallsyms]) => ffffffffb63220d3 perf_output_begin+0x93 ([kernel.kallsyms])
uname 14740 [006] 26795.092638: 1 branches: ffffffffb63220e8 perf_output_begin+0xa8 ([kernel.kallsyms]) => ffffffffb63220ff perf_output_begin+0xbf ([kernel.kallsyms])
uname 14740 [006] 26795.092638: PERF_RECORD_EXIT(14740:14740):(14739:14739)
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb6322119 perf_output_begin+0xd9 ([kernel.kallsyms]) => ffffffffb6322128 perf_output_begin+0xe8 ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb6322146 perf_output_begin+0x106 ([kernel.kallsyms]) => ffffffffb63220ea perf_output_begin+0xaa ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb63220f9 perf_output_begin+0xb9 ([kernel.kallsyms]) => ffffffffb63221ab perf_output_begin+0x16b ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb63221ae perf_output_begin+0x16e ([kernel.kallsyms]) => ffffffffb63221b6 perf_output_begin+0x176 ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb6322202 perf_output_begin+0x1c2 ([kernel.kallsyms]) => ffffffffb6322167 perf_output_begin+0x127 ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb632218c perf_output_begin+0x14c ([kernel.kallsyms]) => ffffffffb631696b perf_event_task_output+0x9b ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb6316990 perf_event_task_output+0xc0 ([kernel.kallsyms]) => ffffffffb61034a0 __task_pid_nr_ns+0x0 ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb61034b7 __task_pid_nr_ns+0x17 ([kernel.kallsyms]) => ffffffffb6194dc0 __rcu_read_lock+0x0 ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb6194de1 __rcu_read_lock+0x21 ([kernel.kallsyms]) => ffffffffb61034bc __task_pid_nr_ns+0x1c ([kernel.kallsyms])
:14740 14740 [006] 26795.092638: 1 branches: ffffffffb6103503 __task_pid_nr_ns+0x63 ([kernel.kallsyms]) => ffffffffb610353b __task_pid_nr_ns+0x9b ([kernel.kallsyms])

This will also affect samples made after PERF_RECORD_EXIT but before
the task finishes exiting.

>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
> tools/perf/builtin-report.c | 7 +++++++
> tools/perf/util/machine.c | 10 +++++++---
> tools/perf/util/symbol_conf.h | 3 ++-
> tools/perf/util/thread.h | 14 ++++++++++++++
> 4 files changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/builtin-report.c b/tools/perf/builtin-report.c
> index dcedfe00f04d..749246817aed 100644
> --- a/tools/perf/builtin-report.c
> +++ b/tools/perf/builtin-report.c
> @@ -1411,6 +1411,13 @@ int cmd_report(int argc, const char **argv)
> if (ret < 0)
> goto exit;
>
> + /*
> + * tasks_mode require access to exited threads to list those that are in
> + * the data file. Off-cpu events are synthesized after other events and
> + * reference exited threads.
> + */
> + symbol_conf.keep_exited_threads = true;
> +
> annotation_options__init(&report.annotation_opts);
>
> ret = perf_config(report__config, &report);
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index 6ca7500e2cf4..5cda47eb337d 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -2157,9 +2157,13 @@ int machine__process_exit_event(struct machine *machine, union perf_event *event
> if (dump_trace)
> perf_event__fprintf_task(event, stdout);
>
> - if (thread != NULL)
> - thread__put(thread);
> -
> + if (thread != NULL) {
> + if (symbol_conf.keep_exited_threads)
> + thread__set_exited(thread, /*exited=*/true);
> + else
> + machine__remove_thread(machine, thread);
> + }
> + thread__put(thread);
> return 0;
> }
>
> diff --git a/tools/perf/util/symbol_conf.h b/tools/perf/util/symbol_conf.h
> index 2b2fb9e224b0..6040286e07a6 100644
> --- a/tools/perf/util/symbol_conf.h
> +++ b/tools/perf/util/symbol_conf.h
> @@ -43,7 +43,8 @@ struct symbol_conf {
> disable_add2line_warn,
> buildid_mmap2,
> guest_code,
> - lazy_load_kernel_maps;
> + lazy_load_kernel_maps,
> + keep_exited_threads;
> const char *vmlinux_name,
> *kallsyms_name,
> *source_prefix,
> diff --git a/tools/perf/util/thread.h b/tools/perf/util/thread.h
> index e79225a0ea46..0df775b5c110 100644
> --- a/tools/perf/util/thread.h
> +++ b/tools/perf/util/thread.h
> @@ -36,13 +36,22 @@ struct thread_rb_node {
> };
>
> DECLARE_RC_STRUCT(thread) {
> + /** @maps: mmaps associated with this thread. */
> struct maps *maps;
> pid_t pid_; /* Not all tools update this */
> + /** @tid: thread ID number unique to a machine. */
> pid_t tid;
> + /** @ppid: parent process of the process this thread belongs to. */
> pid_t ppid;
> int cpu;
> int guest_cpu; /* For QEMU thread */
> refcount_t refcnt;
> + /**
> + * @exited: Has the thread had an exit event. Such threads are usually
> + * removed from the machine's threads but some events/tools require
> + * access to dead threads.
> + */
> + bool exited;
> bool comm_set;
> int comm_len;
> struct list_head namespaces_list;
> @@ -189,6 +198,11 @@ static inline refcount_t *thread__refcnt(struct thread *thread)
> return &RC_CHK_ACCESS(thread)->refcnt;
> }
>
> +static inline void thread__set_exited(struct thread *thread, bool exited)
> +{
> + RC_CHK_ACCESS(thread)->exited = exited;
> +}
> +
> static inline bool thread__comm_set(const struct thread *thread)
> {
> return RC_CHK_ACCESS(thread)->comm_set;