linux-kernel - Re: [PATCH] rcu/torture: Change order of warning and trace dump

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Date:   Tue, 12 Apr 2022 08:25:01 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Benedikt Spranger <b.spranger@...utronix.de>
Subject: Re: [PATCH] rcu/torture: Change order of warning and trace dump

On Mon, Apr 11, 2022 at 11:09:15AM -0700, Paul E. McKenney wrote:
> On Mon, Apr 11, 2022 at 05:19:03PM +0200, Anna-Maria Behnsen wrote:
> > Dumping a big ftrace buffer could lead to a RCU stall. So there is the
> > ftrace buffer and the stall information which needs to be printed. When
> > there is additionaly a WARN_ON() which describes the reason for the ftrace
> > buffer dump and the WARN_ON() is executed _after_ ftrace buffer dump, the
> > information get lost in the middle of the RCU stall information.
> > 
> > Therefore print WARN_ON() message before dumping the ftrace buffer in
> > rcu_torture_writer().
> > 
> > Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> > Reviewed-by: Benedikt Spranger <b.spranger@...utronix.de>
> 
> Hello, Anna-Maria!
> 
> Good point, but we get caught out either way.  Either we take the chance
> of losing the WARN() message as you say, or we take the chance of the
> activity in the WARN() message overwriting needed information in the
> trace buffer.
> 
> Would it work to shut off tracing, do the WARN(), and only then do the
> rcu_ftrace_dump()?

And presumably you are looking at this because your testing is
triggering it.  This WARN() assumes that the system running rcutorture
is otherwise idle.  If you are (say) running kernel builds while also
running rcutorture, then this WARN() is expected behavior.  So if you need
this sort of testing, we need do something like adding another rcutorture
module parameter (background_load?) that suppresses this warning.

							Thanx, Paul

> > ---
> >  kernel/rcu/rcutorture.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 55d049c39608..cbbe37d7701e 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -1287,8 +1287,8 @@ rcu_torture_writer(void *arg)
> >  				if (list_empty(&rcu_tortures[i].rtort_free) &&
> >  				    rcu_access_pointer(rcu_torture_current) !=
> >  				    &rcu_tortures[i]) {
> > -					rcu_ftrace_dump(DUMP_ALL);
> >  					WARN(1, "%s: rtort_pipe_count: %d\n", __func__, rcu_tortures[i].rtort_pipe_count);
> > +					rcu_ftrace_dump(DUMP_ALL);
> >  				}
> >  		if (stutter_waited)
> >  			sched_set_normal(current, oldnice);
> > -- 
> > 2.20.1
> >