Re: [PATCH v5 0/6] perf: Make SIGTRAP and __perf_pending_irq() work on RT.

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Thu, Jul 04, 2024 at 04:45:22PM -0300, Arnaldo Carvalho de Melo wrote:
> On Thu, Jul 04, 2024 at 07:03:34PM +0200, Sebastian Andrzej Siewior wrote:
> > Hi,
> > 
> > Arnaldo reported that "perf test sigtrap" fails on PREEMPT_RT. Sending
> > the signal gets delayed until event_sched_out() which then uses
> > task_work_add() for its delivery. This breaks on PREEMPT_RT because the
> > signal is delivered with disabled preemption.
> > 
> > While looking at this, I also stumbled upon __perf_pending_irq() which
> > requires disabled interrupts but this is not the case on PREEMPT_RT.
> > 
> > This series aim to address both issues while not introducing a new issue
> > at the same time ;)
> > Any testing is appreciated.
> 
> Were should I apply this patch? The v4 series was applied to
> linux-rt-devel/linux-6.10.y-rt IIRC

Looking at linux-rt-devel/linux-6.10.y-rt I see:

commit ca8b27c51f0962f8fb59e5acb23e0af791fb5c04
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Tue Jun 25 10:56:32 2024 +0200

    perf: Update the perf series
    
    This is an all-in-one patch integrating the following changes:
    
    - Merging Frederick's "Fix leaked sigtrap events" series as of v4 which
      is a dependency.
    
    - Update the "perf test sigtrap" fixup to v4 as posted.
    
    Link: https://lore.kernel.org/20240621091601.18227-1-frederic@kernel.org
    Link: https://lore.kernel.org/20240624152732.1231678-1-bigeasy@linutronix.de
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

But I think that is v4, right?

- Arnaldo
>  
> > v4…v5: https://lore.kernel.org/all/20240624152732.1231678-1-bigeasy@linutronix.de/
> >    - Add TWA_NMI_CURRENT as notify mode for task_work_add() and use it.
> >      PeterZ pointed out that the current version is not NMI safe.
> > 
> > v3…v4: https://lore.kernel.org/all/20240322065208.60456-1-bigeasy@linutronix.de/
> >    - Rebased on top of Frederic's series
> >       (https://lore.kernel.org/all/20240621091601.18227-1-frederic@kernel.org)
> >    - Frederick pointed out that perf_pending_task() needs to
> >      perf_swevent_get_recursion_context() in order not to recurse if
> >      something within perf_swevent_.*_recursion_context() triggers a
> >      software event. To address this, the counters have been moved to
> >      the task_struct (#3 + #4) and preemt_disable() has been replaced
> >      with a RCU-read lock (#5).
> >    - The remaning logic same that means the event is pushed to task-work
> >      instead of delivering from IRQ-work. The series was tested with
> >      remove_on_exec as suggested by Marco Elver: On PREEMPT_RT a single
> >      invocation passes, 100 parallel invocations report (for some)
> >      unexpected SIGTRAPs and timeouts. This also observed on !RT
> >      (without the series) with a higher task-count.
> > 
> > v2…v3: https://lore.kernel.org/all/20240312180814.3373778-1-bigeasy@linutronix.de/
> >     - Marco suggested to add a few comments
> >       - Added a comment to __perf_event_overflow() to explain why irq_work
> >         is raised in the in_nmi() case.
> >       - Added a comment to perf_event_exit_event() to explain why the
> >         pending event is deleted.
> > 
> > v1…v2: https://lore.kernel.org/all/20240308175810.2894694-1-bigeasy@linutronix.de/
> >     - Marco pointed me to the testsuite that showed two problems:
> >       - Delayed task_work from NMI / missing events.
> >         Fixed by triggering dummy irq_work to enforce an interrupt for
> > 	the exit-to-userland path which checks task_work
> >       - Increased ref-count on clean up/ during exec.
> >         Mostly addressed by the former change. There is still a window
> > 	if the NMI occurs during execve(). This is addressed by removing
> > 	the task_work before free_event().
> >       The testsuite (remove_on_exec) fails sometimes if the event/
> >       SIGTRAP is sent before the sighandler is installed.
> > 
> > Sebastian
> >