Re: [PATCH rcu 5/6] doc/rcutorture: Add description of rcutorture.stall_cpu_block

[Joel Fernandes]

On Thu, May 11, 2023 at 11:11 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> wrote:
> > >
> > > From: Zqiang <qiang1.zhang@xxxxxxxxx>
> > >
> > > 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"?

Hmm, I guess is not clear to the reader without code reading about why
preempt got disabled. So I would add that last part I mentioned, but I
am Ok either way, it is just a suggestion.

>
> > > +                       Which might or might not be what you want.
> > > +
> >
> > Suggest drop this line ;-).
>
> OK, I will bite.  ;-)
>
> What is your concern with this line?

It is not needed IMO.

thanks,

 - Joel


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