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

[Wen Yang]

On 2024/2/23 00:25, Mathieu Desnoyers wrote:
> On 2024-02-22 11:04, wenyang.linux@xxxxxxxxxxx wrote:
> > From: Wen Yang <wenyang.linux@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >   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);