Re: [PATCH rcu v2] 4/5] rcu-tasks: Move RCU Tasks self-tests to core_initcall()

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Feb 04, 2025 at 11:26:11AM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-01-30 10:53:19 [-0800], Paul E. McKenney wrote:
> > The timer and hrtimer softirq processing has moved to dedicated threads
> > for kernels built with CONFIG_IRQ_FORCED_THREADING=y.  This results in
> > timers not expiring until later in early boot, which in turn causes the
> > RCU Tasks self-tests to hang in kernels built with CONFIG_PROVE_RCU=y,
> > which further causes the entire kernel to hang.  One fix would be to
> > make timers work during this time, but there are no known users of RCU
> > Tasks grace periods during that time, so no justification for the added
> > complexity.  Not yet, anyway.
> > 
> > This commit therefore moves the call to rcu_init_tasks_generic() from
> > kernel_init_freeable() to a core_initcall().  This works because the
> > timer and hrtimer kthreads are created at early_initcall() time.
> 
> Fixes: 49a17639508c3 ("softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.")
> ?

Quite possibly...  I freely confess that I was more focused on the fix
than on the bug's origin.  Would you be willing to try this commit and
its predecessor?

> I played with it and I can reproduce the issue with !RT + threadirqs but
> not with RT (which implies threadirqs).
> Is there anything in RT that avoids the problem?

Not that I know of, but then again I did not try it.  To your point,
I do need to make a -rt rcutorture scenario.  TREE03 has been intended to
approximate this, and it uses the following Kconfig options:

------------------------------------------------------------------------

CONFIG_SMP=y
CONFIG_NR_CPUS=16
CONFIG_PREEMPT_NONE=n
CONFIG_PREEMPT_VOLUNTARY=n
CONFIG_PREEMPT=y
#CHECK#CONFIG_PREEMPT_RCU=y
CONFIG_HZ_PERIODIC=y
CONFIG_NO_HZ_IDLE=n
CONFIG_NO_HZ_FULL=n
CONFIG_RCU_TRACE=y
CONFIG_HOTPLUG_CPU=y
CONFIG_RCU_FANOUT=2
CONFIG_RCU_FANOUT_LEAF=2
CONFIG_RCU_NOCB_CPU=n
CONFIG_DEBUG_LOCK_ALLOC=n
CONFIG_RCU_BOOST=y
CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
CONFIG_RCU_EXPERT=y

------------------------------------------------------------------------

And the following kernel-boot parameters:

------------------------------------------------------------------------

rcutorture.onoff_interval=200 rcutorture.onoff_holdoff=30
rcutree.gp_preinit_delay=12
rcutree.gp_init_delay=3
rcutree.gp_cleanup_delay=3
rcutree.kthread_prio=2
threadirqs
rcutree.use_softirq=0
rcutorture.preempt_duration=10

------------------------------------------------------------------------

Some of these are for RCU's benefit, but what should I change to more
closely approximate a typical real-time deployment?

> Thank you for debugging and the patch. 

And to you for digging into it!

							Thanx, Paul