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

[Paul E. McKenney]

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

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.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9e5bab29685f..eaffe0f8771d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5087,8 +5087,16 @@
 
 	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.
+			Which might or might not be what you want.
+
 
 	rcutorture.stall_cpu_holdoff= [KNL]
 			Time to wait (s) after boot before inducing stall.
-- 
2.40.1