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

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Date:   Tue, 19 Apr 2022 06:44:16 -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 Tue, Apr 19, 2022 at 08:53:38AM +0200, Anna-Maria Behnsen wrote:
> On Tue, 12 Apr 2022, Paul E. McKenney wrote:
> 
> > 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.
> 
> I ran into this while testing with rcutorture kvm script. And I was the
> only one working on the machine. So I do not need a parameter right
> now. I'll come back to you when my testing requirements will change :)

Sounds good, and thank you for the info!

							Thanx, Paul