Date:   Mon, 20 Mar 2023 13:18:34 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Zqiang <qiang1.zhang@intel.com>
Cc:     frederic@kernel.org, joel@joelfernandes.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] rcutorture: Convert
 schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()
Message-ID: <7a414721-25fa-485c-91a5-13d3149073fe@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20230320032422.4010801-1-qiang1.zhang@intel.com>
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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
>