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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: linux-perf-users@vger.kernel.org,
	linux-kernel@vger.kernel.org
Cc: 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>,
	Marco Elver <elver@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH v3 0/4] perf: Make SIGTRAP and __perf_pending_irq() work on RT.
Date: Fri, 22 Mar 2024 07:48:20 +0100
Message-ID: <20240322065208.60456-1-bigeasy@linutronix.de>
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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.

v2=E2=80=A6v3: 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=E2=80=A6v2: 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