linux-kernel - Re: [PATCH v3 3/4] perf: Remove perf_swevent_get_recursion_context() from perf_pending

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Date: Wed, 10 Apr 2024 15:58:40 +0200
From: Frederic Weisbecker <frederic@...nel.org>
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>,
	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 3/4] perf: Remove perf_swevent_get_recursion_context()
 from perf_pending_task().

Le Wed, Apr 10, 2024 at 02:51:26PM +0200, Sebastian Andrzej Siewior a écrit :
> On 2024-04-10 12:38:50 [+0200], Frederic Weisbecker wrote:
> > Some alternatives:
> > 
> > _ Clear event->pending_work = 0 after perf_sigtrap(), preventing an
> > event in there from adding a new task work. We may miss a signal though...
> > 
> > _ Make the recursion context per task on -RT...
> 
> The per-task counter would be indeed the easiest thing to do. But then
> only for task context, right?

It should work for CPU context as well. A context switch shouldn't be
considered as recursion. Hopefully...

> 
> But why would we miss a signal if we clean event->pending_work late?
> Isn't cleaning late same as clearing in
> perf_swevent_put_recursion_context()?

Not exactly. perf_swevent_get_recursion_context() avoids a new software event
altogether from being recorded. It is completely ignored. Whereas clearing late
event->pending_work records new software events but ignores the signal.

> If clearing pending_work late works, I would prefer to avoid yet another
> per-task counter if possible.

Yeah I know :-/

Thanks.

> 
> > > > Thanks.
> 
> Sebastian