linux-kernel - Re: [PATCH v4 0/6] perf: Make SIGTRAP and __perf_pending

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Date: Tue, 25 Jun 2024 15:42:56 +0200
From: Marco Elver <elver@...gle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Adrian Hunter <adrian.hunter@...el.com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Daniel Bristot de Oliveira <bristot@...nel.org>, 
	Frederic Weisbecker <frederic@...nel.org>, Ian Rogers <irogers@...gle.com>, Ingo Molnar <mingo@...hat.com>, 
	Jiri Olsa <jolsa@...nel.org>, Kan Liang <kan.liang@...ux.intel.com>, 
	Mark Rutland <mark.rutland@....com>, Namhyung Kim <namhyung@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 0/6] perf: Make SIGTRAP and __perf_pending_irq() work
 on RT.

On Mon, 24 Jun 2024 at 17:27, Sebastian Andrzej Siewior
<bigeasy@...utronix.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.

Tested-by: Marco Elver <elver@...gle.com>

Ran lots of concurrent copies of the "sigtrap_threads" and
"remove_on_exec" tests (with lockdep on), and it all survived. Fuzzer
is still running but hasn't found anything relevant yet.


> 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
>