Re: [PATCH v9 00/13] unwind_user: x86: Deferred unwinding infrastructure

On Wed, 21 May 2025 08:26:05 +0900
Masami Hiramatsu (Google) [off-list ref] wrote:

quoted

Maybe I asked this before but I don't remember if I got the answer. :)
How does it handle task exits as it won't go to userspace?  I guess it'll
lose user callstacks for exit syscalls and other termination paths.

I just checked, and the good news is that task_work does indeed get called
when a task exits. The bad news is that it happens after do_exit() cleans
up the task's "mm" structure via exit_mm(). Which means that current->mm is
NULL :-p

There's a proposal to move trace_sched_process_exit() to before exit_mm().
If that happens, we could make that tracepoint a "faultable" tracepoint and
then the unwind infrastructure could attach to it and do the unwinding from
that tracepoint.

quoted

Similarly, it will miss user callstacks in the samples at the end of
profiling if the target tasks remain in the kernel (or they sleep).
It looks like a fundamental limitation of the deferred callchains.  

Yes that is a limitation.

Can we use a hybrid approach for this case?
It might be more balanced (from the performance point of view) to save
the full stack in a classic way only in this case, rather than faulting
on process exit or doing file access just to load the sframe.

Another approach is that the tool (like perf) could request to take the
user space stack trace every time a task enters the kernel via a system
call.

-- Steve