linux-kernel - Re: [PATCH v3 2/4] perf: Enqueue SIGTRAP always via task

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Date: Tue, 9 Apr 2024 15:47:29 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
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>,
	Ian Rogers <irogers@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>, Marco Elver <elver@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH v3 2/4] perf: Enqueue SIGTRAP always via task_work.

On 2024-04-09 14:36:51 [+0200], Frederic Weisbecker wrote:
> > That wake_up() within preempt_disable() section breaks on RT.
> 
> Ah, but the wake-up still wants to go inside recursion protection somehow or
> it could generate task_work loop again due to tracepoint events...

okay.

> Although... the wake up occurs only when the event is dead after all...

corner case or not, it has to work, right?

> > How do we go on from here?
> 
> I'd tend to think you need my patchset first because the problems it
> fixes were not easily visible as long as there was an irq work to take
> care of things most of the time. But once you rely on task_work only then
> these become a real problem. Especially the sync against perf_release().

I don't mind rebasing on top of your series. But defaulting to task_work
is not an option then?

RT wise the irq_work is not handled in hardirq because of locks it
acquires and is handled instead in a thread. Depending on the priority
the task (receiving the event) it may run before the irq_work-thread.
Therefore the task_work looked neat because the event would be handled
_before_ the task returned to userland.

Couldn't we either flush _or_ remove the task_work in perf_release()?

> Thanks.
Sebastian