[PATCH v3] doc/rcutorture: Add description of rcutorture.stall_cpu_block

[Zqiang]

For kernels built with CONFIG_PREEMPTION=n and CONFIG_PREEMPT_COUNT=y,
run the RCU stall tests.

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

[   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

Due to invoke schedule_timeout() forces the CPU to go through a
quiescent state, cause RCU stall not appear and also cause scheduling
while atomic complaints. so this commit add description of
rcutorture.stall_cpu_block, it should not to be set in CONFIG_PREEMPTION=n
kernels.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6734d1255682..56f4eb8c7cf2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5053,6 +5053,11 @@
 			Sleep while stalling if set.  This will result
 			in warnings from preemptible RCU in addition
 			to any other stall-related activity.
+			Note that for CONFIG_PREEMPTION=n kernel, if set.
+			will forces the CPU to go through a quiescent state
+			and it can also cause scheduling-while-atomic
+			complaints(CONFIG_PREEMPT_COUNT=y), so should not be set.
+
 
 	rcutorture.stall_cpu_holdoff= [KNL]
 			Time to wait (s) after boot before inducing stall.
-- 
2.25.1