Re: [PATCH rcu 5/6] doc/rcutorture: Add description of rcutorture.stall_cpu_block
From: Paul E. McKenney
Date: Thu May 11 2023 - 14:12:02 EST
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 :-).
> > 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?
> - Joel
> > rcutorture.stall_cpu_holdoff= [KNL]
> > Time to wait (s) after boot before inducing stall.
> > --
> > 2.40.1