doc/rcutorture: Add description of rcutorture.stall_cpu_block

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

This is because the rcutorture.stall_cpu_block=1 module parameter causes
rcu_torture_stall() to invoke schedule_timeout_uninterruptible() within
an RCU read-side critical section.  This in turn results in a quiescent
state (which prevents the stall) and a sleep in an atomic context (which
produces the above splat).

Although this code is operating as designed, the design has proven to
be counterintuitive to many.  This commit therefore updates the description
in kernel-parameters.txt accordingly.

[ paulmck: Apply Joel Fernandes feedback. ]
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -5087,8 +5087,17 @@
 
 	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.
+			Given CONFIG_PREEMPTION=n, this will suppress
+			RCU CPU stall warnings, but will instead result
+			in scheduling-while-atomic splats.
+
+			Use of this module parameter results in splats.
+
 
 	rcutorture.stall_cpu_holdoff= [KNL]
 			Time to wait (s) after boot before inducing stall.