linux-kernel - Re: [PATCH v2 0/4] perf: Make SIGTRAP and __perf_pending

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Date: Wed, 13 Mar 2024 12:23:32 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Marco Elver <elver@...gle.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ian Rogers <irogers@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 0/4] perf: Make SIGTRAP and __perf_pending_irq() work
 on RT.

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@...utronix.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@...gle.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.

- Arnaldo