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

From: Adrian Hunter
Date: Tue Oct 24 2023 - 01:19:40 EST

On 23/10/23 21:49, Ian Rogers wrote:
> On Mon, Oct 23, 2023 at 7:24 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>
>> 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.
>
> Makes sense. Would an appropriate fix be in perf_session__open to set:
> symbol_conf.keep_exited_threads = true;
>
> when:
> perf_header__has_feat(&session->header, HEADER_AUXTRACE)
>
> It is kind of hacky to be changing global state this way, but
> symbol_conf is like that in general.

That should work. Alternatively, could be added to
perf_event__process_auxtrace_info() which would tie it
more directly to auxtrace, and wouldn't have to check
HEADER_AUXTRACE.