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

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Message-ID: <20240410125126.X26tR8tM@linutronix.de>
Date: Wed, 10 Apr 2024 14:51:26 +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 3/4] perf: Remove perf_swevent_get_recursion_context()
 from perf_pending_task().

On 2024-04-10 12:38:50 [+0200], Frederic Weisbecker wrote:
> > Anyway, I misunderstood the concept
> > before. That means we need to keep that counter here and a
> > migrate_disable() is needed to avoid CPU migration which is sad.
> 
> I fear that won't work either. The task is then pinned but another
> task can come up on that CPU and its software events will be ignored...

oh, right.

> 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?

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

> > > Thanks.

Sebastian