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Date:   Thu, 11 May 2023 11:11:31 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org,
        Zqiang <qiang1.zhang@intel.com>
Subject: Re: [PATCH rcu 5/6] doc/rcutorture: Add description of
 rcutorture.stall_cpu_block
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On Wed, May 10, 2023 at 10:47:36PM -0700, Joel Fernandes wrote:
> On Wed, May 10, 2023 at 10:12 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > From: Zqiang <qiang1.zhang@intel.com>
> >
> > If you build a kernel with CONFIG_PREEMPTION=n and CONFIG_PREEMPT_COUNT=y,
> > then run the rcutorture tests specifying stalls as follows:
> >
> > runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4" \
> >         bootparams="console=ttyS0 rcutorture.stall_cpu=30 \
> >         rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_block=1" -d
> >
> > The tests will produce the following splat:
> >
> > [   10.841071] rcu-torture: rcu_torture_stall begin CPU stall
> > [   10.841073] rcu_torture_stall start on CPU 3.
> > [   10.841077] BUG: scheduling while atomic: rcu_torture_sta/66/0x0000000
> > ....
> > [   10.841108] Call Trace:
> > [   10.841110]  <TASK>
> > [   10.841112]  dump_stack_lvl+0x64/0xb0
> > [   10.841118]  dump_stack+0x10/0x20
> > [   10.841121]  __schedule_bug+0x8b/0xb0
> > [   10.841126]  __schedule+0x2172/0x2940
> > [   10.841157]  schedule+0x9b/0x150
> > [   10.841160]  schedule_timeout+0x2e8/0x4f0
> > [   10.841192]  schedule_timeout_uninterruptible+0x47/0x50
> > [   10.841195]  rcu_torture_stall+0x2e8/0x300
> > [   10.841199]  kthread+0x175/0x1a0
> > [   10.841206]  ret_from_fork+0x2c/0x50
> 
> Another way to get rid of the warning would be to replace the
> cur_ops->readlock() with rcu_read_lock(). Though perhaps that will not
> test whether the particular RCU flavor under testing is capable of
> causing a stall :-).

Exactly!

> >         rcutorture.stall_cpu_block= [KNL]
> >                         Sleep while stalling if set.  This will result
> > -                       in warnings from preemptible RCU in addition
> > -                       to any other stall-related activity.
> > +                       in warnings from preemptible RCU in addition to
> > +                       any other stall-related activity.  Note that
> > +                       in kernels built with CONFIG_PREEMPTION=n and
> > +                       CONFIG_PREEMPT_COUNT=y, this parameter will
> > +                       cause the CPU to pass through a quiescent state.
> > +                       Any such quiescent states will suppress RCU CPU
> > +                       stall warnings, but the time-based sleep will
> > +                       also result in scheduling-while-atomic splats.
> 
> Could change last part to "but may also result in
> scheduling-while-atomic splats as preemption might be disabled for
> certain RCU flavors in order to cause the stall".

Is that needed given the earlier "in kernels built with
CONFIG_PREEMPTION=n and CONFIG_PREEMPT_COUNT=y"?

> > +                       Which might or might not be what you want.
> > +
> 
> Suggest drop this line ;-).

OK, I will bite.  ;-)

What is your concern with this line?

							Thanx, Paul

>  - Joel
> 
> >         rcutorture.stall_cpu_holdoff= [KNL]
> >                         Time to wait (s) after boot before inducing stall.
> > --
> > 2.40.1
> >