[PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Zqiang]

For kernels built with enable PREEMPT_NONE and CONFIG_DEBUG_ATOMIC_SLEEP,
running the RCU stall tests.

runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4"
bootparams="nokaslr console=ttyS0 rcutorture.stall_cpu=30
rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_irqsoff=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

The above calltrace occurs in the local_irq_disable/enable() critical
section call schedule_timeout(), and invoke schedule_timeout() also
implies a quiescent state, of course it also fails to trigger RCU stall,
this commit therefore use mdelay() instead of schedule_timeout() to
trigger RCU stall.

Suggested-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
---
 kernel/rcu/rcutorture.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index d06c2da04c34..a08a72bef5f1 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2472,7 +2472,7 @@ static int rcu_torture_stall(void *args)
 #ifdef CONFIG_PREEMPTION
 				preempt_schedule();
 #else
-				schedule_timeout_uninterruptible(HZ);
+				mdelay(jiffies_to_msecs(HZ));
 #endif
 			} else if (stall_no_softlockup) {
 				touch_softlockup_watchdog();
-- 
2.25.1

RE: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Zhuo, Qiuxu]

Hi Qiang,

> From: Zqiang <qiang1.zhang@intel.com>
> Sent: Monday, March 20, 2023 11:24 AM
> To: paulmck@kernel.org; frederic@kernel.org; joel@joelfernandes.org
> Cc: rcu@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible()
> to mdelay() in rcu_torture_stall()
> 
> For kernels built with enable PREEMPT_NONE and

s/enable/enabling/

> CONFIG_DEBUG_ATOMIC_SLEEP, running the RCU stall tests.

s/running/run

> 
> runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4"
> bootparams="nokaslr console=ttyS0 rcutorture.stall_cpu=30
> rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_irqsoff=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
> 
> The above calltrace occurs in the local_irq_disable/enable() critical section
> call schedule_timeout(), and invoke schedule_timeout() also implies a
> quiescent state, of course it also fails to trigger RCU stall, this commit
> therefore use mdelay() instead of schedule_timeout() to trigger RCU stall.

Tweak the commit description above to fix some grammar errors:

    The above call trace occurred in the local_irq_disable/enable() critical section
    when calling schedule_timeout() from rcu_torture_stall(). Invoking schedule_timeout() 
    also implies a quiescent state, of course, it also fails to trigger RCU stall. This commit, 
    therefore, uses mdelay() instead of schedule_timeout() to trigger the RCU stall.

> Suggested-by: Joel Fernandes <joel@joelfernandes.org>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>

I didn't reproduce the call trace after applying your patch.
So, with the above minor fixes, then

    Tested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>

Thanks
-Qiuxu

> ---
>  kernel/rcu/rcutorture.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c index
> d06c2da04c34..a08a72bef5f1 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -2472,7 +2472,7 @@ static int rcu_torture_stall(void *args)  #ifdef
> CONFIG_PREEMPTION
>  				preempt_schedule();
>  #else
> -				schedule_timeout_uninterruptible(HZ);
> +				mdelay(jiffies_to_msecs(HZ));
>  #endif
>  			} else if (stall_no_softlockup) {
>  				touch_softlockup_watchdog();
> --
> 2.25.1

Re: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Paul E. McKenney]

On Mon, Mar 20, 2023 at 11:24:22AM +0800, Zqiang wrote:
> For kernels built with enable PREEMPT_NONE and CONFIG_DEBUG_ATOMIC_SLEEP,
> running the RCU stall tests.
> 
> runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4"
> bootparams="nokaslr console=ttyS0 rcutorture.stall_cpu=30
> rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_irqsoff=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
> 
> The above calltrace occurs in the local_irq_disable/enable() critical
> section call schedule_timeout(), and invoke schedule_timeout() also
> implies a quiescent state, of course it also fails to trigger RCU stall,
> this commit therefore use mdelay() instead of schedule_timeout() to
> trigger RCU stall.
> 
> Suggested-by: Joel Fernandes <joel@joelfernandes.org>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
>  kernel/rcu/rcutorture.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index d06c2da04c34..a08a72bef5f1 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -2472,7 +2472,7 @@ static int rcu_torture_stall(void *args)

Right here there is:

			if (stall_cpu_block) {

In other words, the rcutorture.stall_cpu_block module parameter says to
block, even if it is a bad thing to do.  The point of this is to verify
the error messages that are supposed to be printed on the console when
this happens.

>  #ifdef CONFIG_PREEMPTION
>  				preempt_schedule();
>  #else
> -				schedule_timeout_uninterruptible(HZ);
> +				mdelay(jiffies_to_msecs(HZ));

So this really needs to stay schedule_timeout_uninterruptible(HZ).

So should there be a change to kernel-parameters.txt to make it
more clear that this is intended behavior?

						Thanx, Paul

>  #endif
>  			} else if (stall_no_softlockup) {
>  				touch_softlockup_watchdog();
> -- 
> 2.25.1
> 

RE: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Zhang, Qiang1]

> For kernels built with enable PREEMPT_NONE and CONFIG_DEBUG_ATOMIC_SLEEP,
> running the RCU stall tests.
> 
> runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4"
> bootparams="nokaslr console=ttyS0 rcutorture.stall_cpu=30
> rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_irqsoff=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
> 
> The above calltrace occurs in the local_irq_disable/enable() critical
> section call schedule_timeout(), and invoke schedule_timeout() also
> implies a quiescent state, of course it also fails to trigger RCU stall,
> this commit therefore use mdelay() instead of schedule_timeout() to
> trigger RCU stall.
> 
> Suggested-by: Joel Fernandes <joel@joelfernandes.org>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
>  kernel/rcu/rcutorture.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index d06c2da04c34..a08a72bef5f1 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -2472,7 +2472,7 @@ static int rcu_torture_stall(void *args)
>
>Right here there is:
>
>			if (stall_cpu_block) {
>
>In other words, the rcutorture.stall_cpu_block module parameter says to
>block, even if it is a bad thing to do.  The point of this is to verify
>the error messages that are supposed to be printed on the console when
>this happens.
>
>  #ifdef CONFIG_PREEMPTION
>  				preempt_schedule();
>  #else
> -				schedule_timeout_uninterruptible(HZ);
> +				mdelay(jiffies_to_msecs(HZ));
>
>So this really needs to stay schedule_timeout_uninterruptible(HZ).

But invoke schedule_timeout_uninterruptible(HZ) implies a quiescent state, 
this will not cause an RCU stall to occur, and still in the RCU read critical section(PREEMPT_COUNT=y).

It didn't happen RCU stall when I tested with the following parameters for 
rcutorture.stall_cpu=30
rcutorture.stall_no_softlockup=1
rcutorture.stall_cpu_irqsoff=1
rcutorture.stall_cpu_block=1


Thanks
Zqiang

>
>So should there be a change to kernel-parameters.txt to make it
>more clear that this is intended behavior?
>
>						Thanx, Paul
>
>  #endif
>  			} else if (stall_no_softlockup) {
>  				touch_softlockup_watchdog();
> -- 
> 2.25.1
> 

Re: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Paul E. McKenney]

On Mon, Mar 20, 2023 at 11:05:17PM +0000, Zhang, Qiang1 wrote:
> > For kernels built with enable PREEMPT_NONE and CONFIG_DEBUG_ATOMIC_SLEEP,
> > running the RCU stall tests.
> > 
> > runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4"
> > bootparams="nokaslr console=ttyS0 rcutorture.stall_cpu=30
> > rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_irqsoff=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
> > 
> > The above calltrace occurs in the local_irq_disable/enable() critical
> > section call schedule_timeout(), and invoke schedule_timeout() also
> > implies a quiescent state, of course it also fails to trigger RCU stall,
> > this commit therefore use mdelay() instead of schedule_timeout() to
> > trigger RCU stall.
> > 
> > Suggested-by: Joel Fernandes <joel@joelfernandes.org>
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > ---
> >  kernel/rcu/rcutorture.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index d06c2da04c34..a08a72bef5f1 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -2472,7 +2472,7 @@ static int rcu_torture_stall(void *args)
> >
> >Right here there is:
> >
> >			if (stall_cpu_block) {
> >
> >In other words, the rcutorture.stall_cpu_block module parameter says to
> >block, even if it is a bad thing to do.  The point of this is to verify
> >the error messages that are supposed to be printed on the console when
> >this happens.
> >
> >  #ifdef CONFIG_PREEMPTION
> >  				preempt_schedule();
> >  #else
> > -				schedule_timeout_uninterruptible(HZ);
> > +				mdelay(jiffies_to_msecs(HZ));
> >
> >So this really needs to stay schedule_timeout_uninterruptible(HZ).
> 
> But invoke schedule_timeout_uninterruptible(HZ) implies a quiescent state, 
> this will not cause an RCU stall to occur, and still in the RCU read critical section(PREEMPT_COUNT=y).
> 
> It didn't happen RCU stall when I tested with the following parameters for 
> rcutorture.stall_cpu=30
> rcutorture.stall_no_softlockup=1
> rcutorture.stall_cpu_irqsoff=1
> rcutorture.stall_cpu_block=1

Understood.  If you want that RCU CPU stall in a CONFIG_PREEMPTION=n
kernel, you should not use rcutorture.stall_cpu_block=1.

In a CONFIG_PREEMPTION=y kernel, rcutorture.stall_cpu_block=1 forces
the grace period to be stalled on a task rather than a CPU, exercising
a different part of the RCU CPU stall warning code.

In a CONFIG_PREEMPTION=n kernel, using rcutorture.stall_cpu_block=1
forces the CPU to go through a quiescent state, as you say.  It can
also cause lockdep and scheduling-while-atomic complaints, depending on
exactly what type of RCU reader is in effect.

So these are test-the-diagnostics parameters.  The mdelay() instead
makes rcutorture.stall_cpu_block=1 do the same thing as does
rcutorture.stall_cpu_block=0 for CONFIG_PREEMPTION=n kernels, right?

							Thanx, Paul

> Thanks
> Zqiang
> 
> >
> >So should there be a change to kernel-parameters.txt to make it
> >more clear that this is intended behavior?
> >
> >						Thanx, Paul
> >
> >  #endif
> >  			} else if (stall_no_softlockup) {
> >  				touch_softlockup_watchdog();
> > -- 
> > 2.25.1
> > 

RE: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Zhang, Qiang1]

> > For kernels built with enable PREEMPT_NONE and CONFIG_DEBUG_ATOMIC_SLEEP,
> > running the RCU stall tests.
> > 
> > runqemu kvm slirp nographic qemuparams="-m 1024 -smp 4"
> > bootparams="nokaslr console=ttyS0 rcutorture.stall_cpu=30
> > rcutorture.stall_no_softlockup=1 rcutorture.stall_cpu_irqsoff=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
> > 
> > The above calltrace occurs in the local_irq_disable/enable() critical
> > section call schedule_timeout(), and invoke schedule_timeout() also
> > implies a quiescent state, of course it also fails to trigger RCU stall,
> > this commit therefore use mdelay() instead of schedule_timeout() to
> > trigger RCU stall.
> > 
> > Suggested-by: Joel Fernandes <joel@joelfernandes.org>
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > ---
> >  kernel/rcu/rcutorture.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index d06c2da04c34..a08a72bef5f1 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -2472,7 +2472,7 @@ static int rcu_torture_stall(void *args)
> >
> >Right here there is:
> >
> >			if (stall_cpu_block) {
> >
> >In other words, the rcutorture.stall_cpu_block module parameter says to
> >block, even if it is a bad thing to do.  The point of this is to verify
> >the error messages that are supposed to be printed on the console when
> >this happens.
> >
> >  #ifdef CONFIG_PREEMPTION
> >  				preempt_schedule();
> >  #else
> > -				schedule_timeout_uninterruptible(HZ);
> > +				mdelay(jiffies_to_msecs(HZ));
> >
> >So this really needs to stay schedule_timeout_uninterruptible(HZ).
> 
> But invoke schedule_timeout_uninterruptible(HZ) implies a quiescent state, 
> this will not cause an RCU stall to occur, and still in the RCU read critical section(PREEMPT_COUNT=y).
> 
> It didn't happen RCU stall when I tested with the following parameters for 
> rcutorture.stall_cpu=30
> rcutorture.stall_no_softlockup=1
> rcutorture.stall_cpu_irqsoff=1
> rcutorture.stall_cpu_block=1
>
>Understood.  If you want that RCU CPU stall in a CONFIG_PREEMPTION=n
>kernel, you should not use rcutorture.stall_cpu_block=1.
>
>In a CONFIG_PREEMPTION=y kernel, rcutorture.stall_cpu_block=1 forces
>the grace period to be stalled on a task rather than a CPU, exercising
>a different part of the RCU CPU stall warning code.
>
>In a CONFIG_PREEMPTION=n kernel, using rcutorture.stall_cpu_block=1
>forces the CPU to go through a quiescent state, as you say.  It can
>also cause lockdep and scheduling-while-atomic complaints, depending on
>exactly what type of RCU reader is in effect.
>
>So these are test-the-diagnostics parameters.  The mdelay() instead
>makes rcutorture.stall_cpu_block=1 do the same thing as does
>rcutorture.stall_cpu_block=0 for CONFIG_PREEMPTION=n kernels, right?

Yes,   maybe  we can increase the description of the stall_cpu_block in kernel-parameters.txt.


>
>							Thanx, Paul
>
> Thanks
> Zqiang
> 
> >
> >So should there be a change to kernel-parameters.txt to make it
> >more clear that this is intended behavior?

Agree

Thanks
Zqiang

> >
> >						Thanx, Paul
> >
> >  #endif
> >  			} else if (stall_no_softlockup) {
> >  				touch_softlockup_watchdog();
> > -- 
> > 2.25.1
> > 

RE: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()

[Zhuo, Qiuxu]

> From: Paul E. McKenney <paulmck@kernel.org>
> [...]
> > But invoke schedule_timeout_uninterruptible(HZ) implies a quiescent
> > state, this will not cause an RCU stall to occur, and still in the RCU read
> critical section(PREEMPT_COUNT=y).
> >
> > It didn't happen RCU stall when I tested with the following parameters
> > for
> > rcutorture.stall_cpu=30
> > rcutorture.stall_no_softlockup=1
> > rcutorture.stall_cpu_irqsoff=1
> > rcutorture.stall_cpu_block=1
> 
> Understood.  If you want that RCU CPU stall in a CONFIG_PREEMPTION=n
> kernel, you should not use rcutorture.stall_cpu_block=1.
>

Verified.

if rcutorture.stall_cpu_block=0, it can trigger the expected RCU CPU stall for either 
torture_type=srcu or torture_type=rcu. 
 
> In a CONFIG_PREEMPTION=y kernel, rcutorture.stall_cpu_block=1 forces the
> grace period to be stalled on a task rather than a CPU, exercising a different
> part of the RCU CPU stall warning code.
> 
> In a CONFIG_PREEMPTION=n kernel, using rcutorture.stall_cpu_block=1
> forces the CPU to go through a quiescent state, as you say.  It can also cause
> lockdep and scheduling-while-atomic complaints, depending on exactly what
> type of RCU reader is in effect.
> 

Verified.

If rcutorture.stall_cpu_block=1:
    There were lockdep and scheduling-while-atomic complaints for torture_type=rcu.
    No lockdep and scheduling-while-atomic complaints for torture_type=srcu.

> So these are test-the-diagnostics parameters.  The mdelay() instead makes
> rcutorture.stall_cpu_block=1 do the same thing as does
> rcutorture.stall_cpu_block=0 for CONFIG_PREEMPTION=n kernels, right?

Good to know that these are test-the-diagnostics parameters and their expected behaviors.  ;-)
Thanks!

-Qiuxu

> 							Thanx, Paul