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

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Date: Tue, 9 Apr 2024 15:33:36 +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 2024-04-09 14:00:49 [+0200], Frederic Weisbecker wrote:
> Le Tue, Apr 09, 2024 at 12:54:05PM +0200, Sebastian Andrzej Siewior a écrit :
> > 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. 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.

> Thanks.

Sebastian