From:   "Zhang, Qiang1" <qiang1.zhang@intel.com>
To:     "paulmck@kernel.org" <paulmck@kernel.org>
CC:     "frederic@kernel.org" <frederic@kernel.org>,
        "joel@joelfernandes.org" <joel@joelfernandes.org>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2] rcutorture: Convert schedule_timeout_uninterruptible()
 to mdelay() in rcu_torture_stall()
Thread-Topic: [PATCH v2] rcutorture: Convert
 schedule_timeout_uninterruptible() to mdelay() in rcu_torture_stall()
Thread-Index: AQHZWtris8fTdEhd602c/GqMXMcxwK8EHJQAgAAryuA=
Date:   Mon, 20 Mar 2023 23:05:17 +0000
Message-ID: <PH0PR11MB58806032CE88C9E6BE61059CDA809@PH0PR11MB5880.namprd11.prod.outlook.com>
References: <20230320032422.4010801-1-qiang1.zhang@intel.com>
 <7a414721-25fa-485c-91a5-13d3149073fe@paulmck-laptop>
In-Reply-To: <7a414721-25fa-485c-91a5-13d3149073fe@paulmck-laptop>
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> For kernels built with enable PREEMPT_NONE and CONFIG_DEBUG_ATOMIC_SLEEP,
> running the RCU stall tests.
>=20
> runqemu kvm slirp nographic qemuparams=3D"-m 1024 -smp 4"
> bootparams=3D"nokaslr console=3DttyS0 rcutorture.stall_cpu=3D30
> rcutorture.stall_no_softlockup=3D1 rcutorture.stall_cpu_irqsoff=3D1
> rcutorture.stall_cpu_block=3D1" -d
>=20
> [   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
>=20
> 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.
>=20
> 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(-)
>=20
> 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,=
=20
this will not cause an RCU stall to occur, and still in the RCU read critic=
al section(PREEMPT_COUNT=3Dy).

It didn't happen RCU stall when I tested with the following parameters for=
=20
rcutorture.stall_cpu=3D30
rcutorture.stall_no_softlockup=3D1
rcutorture.stall_cpu_irqsoff=3D1
rcutorture.stall_cpu_block=3D1


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();
> --=20
> 2.25.1
>=20