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Date: Wed, 10 Apr 2024 12:38:50 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	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>,
	Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [PATCH v3 3/4] perf: Remove perf_swevent_get_recursion_context()
 from perf_pending_task().
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Le Tue, Apr 09, 2024 at 03:33:36PM +0200, Sebastian Andrzej Siewior a ?crit :
> On 2024-04-09 14:00:49 [+0200], Frederic Weisbecker wrote:
> > Le Tue, Apr 09, 2024 at 12:54:05PM +0200, Sebastian Andrzej Siewior a ?crit :
> > > On 2024-04-09 12:35:46 [+0200], Frederic Weisbecker wrote:
> > > > > > > @@ -6800,10 +6792,6 @@ static void perf_pending_task(struct callback_head *head)
> > > > > > >  		local_dec(&event->ctx->nr_pending);
> > > > > > >  	}
> > > > > > >  
> > > > > > > -	if (rctx >= 0)
> > > > > > > -		perf_swevent_put_recursion_context(rctx);
> > > > > > > -	preempt_enable_notrace();
> > > > > > 
> > > > > > Well, if a software event happens during perf_sigtrap(), the task work
> > > > > > may be requeued endlessly and the task may get stuck in task_work_run()...
> > > > > 
> > > > > The last time I checked it had no users in the task context. How would
> > > > > that happen?
> > > > 
> > > > I guess many tracepoint events would do the trick. Such as trace_lock_acquire()
> > > > for example.
> > > 
> > > So the perf_trace_buf_alloc() is invoked from that trace point and
> > > avoids the recursion. And any trace event from within perf_sigtrap()
> > > would trigger the endless loop?
> > 
> > No sure I'm following:
> > 
> > 1) event->perf_event_overflow() -> task_work_add()
> > //return to userspace
> > 2) task_work_run() -> perf_pending_task() -> perf_sigtrap() -> tracepoint event
> >    -> perf_event_overflow() -> task_work_add()
> > 3) task_work_run() -> perf_pending_task() -> etc...
> >
> > What am I missing?
> 
> Yes, that is what I tried to say.

Oh ok.. :o)

> 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...

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...

> > Thanks.
> 
> Sebastian