linux-kernel - Re: [PATCH] coredump debugging: add a tracepoint to report the coredumping

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Date: Thu, 22 Feb 2024 00:00:55 +0800
From: Wen Yang <wenyang.linux@...mail.com>
To: Steven Rostedt <rostedt@...dmis.org>, Oleg Nesterov <oleg@...hat.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Mel Gorman <mgorman@...hsingularity.net>,
 Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] coredump debugging: add a tracepoint to report the
 coredumping



On 2024/2/20 01:28, Steven Rostedt wrote:
> On Mon, 19 Feb 2024 18:00:38 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
> 
>>> void __noreturn do_exit(long code)
>>> {
>>> 	struct task_struct *tsk = current;
>>> 	int group_dead;
>>>
>>> [...]
>>> 	acct_collect(code, group_dead);
>>> 	if (group_dead)
>>> 		tty_audit_exit();
>>> 	audit_free(tsk);
>>>
>>> 	tsk->exit_code = code;
>>> 	taskstats_exit(tsk, group_dead);
>>>
>>> 	exit_mm();
>>>
>>> 	if (group_dead)
>>> 		acct_process();
>>> 	trace_sched_process_exit(tsk);
>>>
>>> There's a lot that happens before we trigger the above event.
>>
>> and a lot after.
> 
> True. There really isn't a meaningful location here is there?
> 
> I actually use this tracepoint in my pid tracing.
> 
> The set_ftrace_pid and set_event_pid from /sys/kernel/tracing will add and
> remove PIDs if the options function-fork or event-fork are set respectively.
> 
> I hook to the sched_process_fork tracepoint to add new PIDs if the parent
> pid is already in one of the files, and remove a PID via the
> sched_process_exit function.
> 
> Honestly, if anything, it should probably be moved down right next to
> () (I never understood why  needed its own hooks
> and not just use tracepoints).
> 

Perhaps it's just because perf appeared earlier, and it doesn't rely on 
TRACEPOINTS.
It is indeed reasonable to replace perf_event_exit_task() with 
TRACEPOINT, and we are willing to try to modify it. It will require some 
work and time.

--
Best wishes,
Wen

>>
>> To me the current placement of trace_sched_process_exit() looks absolutely
>> random.
> 
> Agreed.
> 
>>
>>> I could
>>> imagine that there are users expecting those actions to have taken place by
>>> the time the event triggered. Like the "exit_mm()" call, as well as many
>>> others.
>>>
>>> I would be leery of moving that tracepoint.
>>
>> And I agree. I am always scared of every user-visible change, simply
>> because it is user-visbible.
>>
>> If it was not clear, I didn't try to nack this patch. I simply do not know
>> how people use the tracepoints and for what. Apart from debugging.
>>
>> But if we add the new one into coredump_task_exit(), then we probably want
>> another one in ptrace_event(PTRACE_EVENT_EXIT) ? It too can "take some time"
>> before the exiting task actually exits.
>>
>> So I think this needs some discussion, and the changelog should probably say
>> more.
>>
>> In short: I am glad you are here, I leave this to you and Wen ;)
> 
> I still would like to have your input too ;-)
> 
> -- Steve