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References: <20240312180814.3373778-1-bigeasy@linutronix.de>
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From: Marco Elver <elver@google.com>
Date: Wed, 13 Mar 2024 15:35:27 +0100
Message-ID: <CANpmjNMYGa46pRQUOfzTa_FRvftOGqg+UDeD_B-tbZgYw-MWww@mail.gmail.com>
Subject: Re: [PATCH v2 0/4] perf: Make SIGTRAP and __perf_pending_irq() work
 on RT.
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Adrian Hunter <adrian.hunter@intel.com>, 
	Alexander Shishkin <alexander.shishkin@linux.intel.com>, 
	Arnaldo Carvalho de Melo <acme@kernel.org>, Ian Rogers <irogers@google.com>, Ingo Molnar <mingo@redhat.com>, 
	Jiri Olsa <jolsa@kernel.org>, Mark Rutland <mark.rutland@arm.com>, 
	Namhyung Kim <namhyung@kernel.org>, Peter Zijlstra <peterz@infradead.org>, 
	Thomas Gleixner <tglx@linutronix.de>
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On Tue, 12 Mar 2024 at 19:08, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> 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=E2=80=A6v2: https://lore.kernel.org/all/20240308175810.2894694-1-bigea=
sy@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.

Thanks,
-- Marco