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Message-ID: <Z-6TDh1MUT49lvjk@gmail.com>
Date: Thu, 3 Apr 2025 15:54:22 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, peterz@...radead.org,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...a.com, mhocko@...nel.org, rostedt@...dmis.org,
oleg@...hat.com, brauner@...nel.org, glider@...gle.com,
mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
akpm@...ux-foundation.org
Subject: Re: [PATCH v2] exit: move and extend sched_process_exit() tracepoint
* Andrii Nakryiko <andrii@...nel.org> wrote:
> It is useful to be able to access current->mm at task exit to, say,
> record a bunch of VMA information right before the task exits (e.g., for
> stack symbolization reasons when dealing with short-lived processes that
> exit in the middle of profiling session). Currently,
> trace_sched_process_exit() is triggered after exit_mm() which resets
> current->mm to NULL making this tracepoint unsuitable for inspecting
> and recording task's mm_struct-related data when tracing process
> lifetimes.
>
> There is a particularly suitable place, though, right after
> taskstats_exit() is called, but before we do exit_mm() and other
> exit_*() resource teardowns. taskstats performs a similar kind of
> accounting that some applications do with BPF, and so co-locating them
> seems like a good fit. So that's where trace_sched_process_exit() is
> moved with this patch.
>
> Also, existing trace_sched_process_exit() tracepoint is notoriously
> missing `group_dead` flag that is certainly useful in practice and some
> of our production applications have to work around this. So plumb
> `group_dead` through while at it, to have a richer and more complete
> tracepoint.
>
> Note that we can't use sched_process_template anymore, and so we use
> TRACE_EVENT()-based tracepoint definition.
But all the field names and
> order, as well as assign and output logic remain intact. We just add one
> extra field at the end in backwards-compatible way.
>
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
> include/trace/events/sched.h | 28 +++++++++++++++++++++++++---
> kernel/exit.c | 2 +-
> 2 files changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 8994e97d86c1..05a14f2b35c3 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -328,9 +328,31 @@ DEFINE_EVENT(sched_process_template, sched_process_free,
> /*
> * Tracepoint for a task exiting:
> */
> -DEFINE_EVENT(sched_process_template, sched_process_exit,
> - TP_PROTO(struct task_struct *p),
> - TP_ARGS(p));
> +TRACE_EVENT(sched_process_exit,
> +
> + TP_PROTO(struct task_struct *p, bool group_dead),
> +
> + TP_ARGS(p, group_dead),
> +
> + TP_STRUCT__entry(
> + __array( char, comm, TASK_COMM_LEN )
> + __field( pid_t, pid )
> + __field( int, prio )
> + __field( bool, group_dead )
> + ),
> +
> + TP_fast_assign(
> + memcpy(__entry->comm, p->comm, TASK_COMM_LEN);
> + __entry->pid = p->pid;
> + __entry->prio = p->prio; /* XXX SCHED_DEADLINE */
> + __entry->group_dead = group_dead;
> + ),
> +
> + TP_printk("comm=%s pid=%d prio=%d group_dead=%s",
> + __entry->comm, __entry->pid, __entry->prio,
> + __entry->group_dead ? "true" : "false"
> + )
This feels really fragile, could you please at least add a comment that
points out that this is basically an extension of
sched_process_template, and that it should remain a subset of it, or
something to that end?
Thanks,
Ingo
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