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

Arnaldo Carvalho de Melo
2024-03-13T18:14:31Z

On Wed, Mar 13, 2024 at 12:23:32PM -0300, Arnaldo Carvalho de Melo wrote:
> On Wed, Mar 13, 2024 at 03:35:27PM +0100, Marco Elver wrote:
> > On Tue, 12 Mar 2024 at 19:08, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> > > 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@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.
>
> > Tested-by: Marco Elver <elver@google.com>
>
> > 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.

> Right, I'm testing with the full 'perf test' suite now.

'perf test' doesn't show any regression, now I'm running Vince Weaver's
https://github.com/deater/perf_event_tests, storing the results with
this patchset and then without, to do a diff, lets see...

- Arnaldo