Re: [PATCH v2 0/4] perf: Make SIGTRAP and __perf_pending_irq() work on RT.

[Marco Elver]

On Tue, 12 Mar 2024 at 19:08, Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> 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.
>
> v1…v2: https://lore.kernel.org/all/20240308175810.2894694-1-bigeasy@xxxxxxxxxxxxx/
>     - 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.

Tested-by: Marco Elver <elver@xxxxxxxxxx>

It does pass the tests in tools/testing/selftests/perf_events (non-RT
kernel, lockdep enabled). But I do recall this being a particularly
sharp corner of perf, so any additional testing and review here is
useful.

Thanks,
-- Marco