linux-kernel - Re: [PATCH 1/2] exit: add a tracepoint for profiling a task that is starting to exit

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Date: Fri, 23 Feb 2024 13:17:50 +0800
From: Wen Yang <wenyang.linux@...mail.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Ingo Molnar <mingo@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Mel Gorman
 <mgorman@...hsingularity.net>, Peter Zijlstra <peterz@...radead.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] exit: add a tracepoint for profiling a task that is
 starting to exit



On 2024/2/23 00:25, Mathieu Desnoyers wrote:
> On 2024-02-22 11:04, wenyang.linux@...mail.com wrote:
>> From: Wen Yang <wenyang.linux@...mail.com>
>>
>> Currently coredump_task_exit() takes some time to wait for the generation
>> of the dump file. But if the user-space wants to receive a notification
>> as soon as possible it maybe inconvenient.
>>
>> Commit 2d4bcf886e42 ("exit: Remove profile_task_exit & profile_munmap")
>> simplified the code, but also removed profile_task_exit(), which may
>> prevent third-party kernel modules from detecting process exits timely.
>>
>> Add the new trace_sched_profile_task_exit() this way a user-space monitor
>> could detect the exits and potentially make some preparations in advance.
> 
> I don't see any explanation justifying adding an extra tracepoint
> rather than just moving trace_sched_process_exit() earlier in do_exit().
> 
> Why is moving trace_sched_process_exit() earlier in do_exit() an issue,
> considering that any tracer interested in knowing the point where a task
> is really reclaimed (from zombie state) is trace_sched_process_free()
> called from delayed_put_task_struct() ?
> 
> Thanks,
> 
> Mathieu
> 

Thanks.
We will make the modifications according to your suggestions.

--
Best wishes,
Wen

>>
>> Signed-off-by: Wen Yang <wenyang.linux@...mail.com>
>> Cc: Oleg Nesterov <oleg@...hat.com>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Cc: Masami Hiramatsu <mhiramat@...nel.org>
>> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: linux-kernel@...r.kernel.org
>> ---
>>   include/trace/events/sched.h | 28 ++++++++++++++++++++++++++++
>>   kernel/exit.c                |  1 +
>>   2 files changed, 29 insertions(+)
>>
>> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
>> index dbb01b4b7451..750b2f0bdf69 100644
>> --- a/include/trace/events/sched.h
>> +++ b/include/trace/events/sched.h
>> @@ -341,6 +341,34 @@ DEFINE_EVENT(sched_process_template, 
>> sched_wait_task,
>>       TP_PROTO(struct task_struct *p),
>>       TP_ARGS(p));
>> +/*
>> + * Tracepoint for profiling a task that is starting to exit:
>> + */
>> +TRACE_EVENT(sched_profile_task_exit,
>> +
>> +    TP_PROTO(struct task_struct *task, long code),
>> +
>> +    TP_ARGS(task, code),
>> +
>> +    TP_STRUCT__entry(
>> +        __array(    char,    comm,    TASK_COMM_LEN    )
>> +        __field(    pid_t,    pid            )
>> +        __field(    int,    prio            )
>> +        __field(    long,    code            )
>> +    ),
>> +
>> +    TP_fast_assign(
>> +        memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
>> +        __entry->pid        = task->pid;
>> +        __entry->prio        = task->prio;
>> +        __entry->code        = code;
>> +    ),
>> +
>> +    TP_printk("comm=%s pid=%d prio=%d exit_code=0x%lx",
>> +          __entry->comm, __entry->pid, __entry->prio,
>> +          __entry->code)
>> +);
>> +
>>   /*
>>    * Tracepoint for a waiting task:
>>    */
>> diff --git a/kernel/exit.c b/kernel/exit.c
>> index 493647fd7c07..f675f879a1b2 100644
>> --- a/kernel/exit.c
>> +++ b/kernel/exit.c
>> @@ -826,6 +826,7 @@ void __noreturn do_exit(long code)
>>       WARN_ON(tsk->plug);
>> +    trace_sched_profile_task_exit(tsk, code);
>>       kcov_task_exit(tsk);
>>       kmsan_task_exit(tsk);
>