Re: [RESEND][PATCH v15 0/4] perf: Support the deferred unwinding infrastructure

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 18, 2025 at 11:18:53AM -0400, Steven Rostedt wrote:
> On Thu, 18 Sep 2025 13:46:10 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > So I started looking at this, but given I never seen the deferred unwind
> > bits that got merged I have to look at that first.
> > 
> > Headers want something like so.. Let me read the rest.
> > 
> > ---
> >  include/linux/unwind_deferred.h       | 38 +++++++++++++++++++----------------
> >  include/linux/unwind_deferred_types.h |  2 ++
> >  2 files changed, 23 insertions(+), 17 deletions(-)
> 
> Would you like to send a formal patch with this? I'd actually break it into
> two patches. One to clean up the long lines, and the other to change the
> logic.

Sure, I'll collect the lot while I go through it and whip something up
when I'm done. For now, I'll just shoot a few questions your way.


So we have:

do_syscall_64()
  ... do stuff ...
  syscall_exit_to_user_mode(regs)
    syscall_exit_to_user_mode_work(regs)
      syscall_exit_work()
      exit_to_user_mode_prepare()
        exit_to_user_mode_loop()
	  retume_user_mode_work()
	    task_work_run()
    exit_to_user_mode()
      unwind_reset_info();
      user_enter_irqoff();
      arch_exit_to_user_mode();
      lockdep_hardirqs_on();
  SYSRET/IRET


and

DEFINE_IDTENTRY*()
  irqentry_enter();
  ... stuff ...
  irqentry_exit()
    irqentry_exit_to_user_mode()
      exit_to_user_mode_prepare()
        exit_to_user_mode_loop();
	  retume_user_mode_work()
	    task_work_run()
      exit_to_user_mode()
        unwind_reset_info();
	...
  IRET

Now, task_work_run() is in the exit_to_user_mode_loop() which is notably
*before* exit_to_user_mode() which does the unwind_reset_info().

What happens if we get an NMI requesting an unwind after
unwind_reset_info() while still very much being in the kernel on the way
out?


What is the purpose of unwind_deferred_task_exit()? This is called from
do_exit(), only slightly before it does exit_task_work(), which runs all
pending task_work. Is there something that justifies the manual run and
cancel instead of just leaving it sit in task_work an having it run
naturally? If so, that most certainly deserves a comment.


A similar question for unwind_task_free(), where exactly is it relevant?
Where does it acquire a task_work that is not otherwise already ran on
exit?