From:   "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>
To:     "paulmck@kernel.org" <paulmck@kernel.org>,
        "Zhang, Qiang1" <qiang1.zhang@intel.com>
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: AQHZWtsElwxz/KN2NEOd8ANQA4i0X68EHJQAgAAulYCAAAh2AIAAmvHQ
Date:   Tue, 21 Mar 2023 12:27:14 +0000
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> 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=3Dy).
> >
> > It didn't happen RCU stall when I tested with the following parameters
> > for
> > rcutorture.stall_cpu=3D30
> > rcutorture.stall_no_softlockup=3D1
> > rcutorture.stall_cpu_irqsoff=3D1
> > rcutorture.stall_cpu_block=3D1
>=20
> Understood.  If you want that RCU CPU stall in a CONFIG_PREEMPTION=3Dn
> kernel, you should not use rcutorture.stall_cpu_block=3D1.
>

Verified.

if rcutorture.stall_cpu_block=3D0, it can trigger the expected RCU CPU stal=
l for either=20
torture_type=3Dsrcu or torture_type=3Drcu.=20
=20
> In a CONFIG_PREEMPTION=3Dy kernel, rcutorture.stall_cpu_block=3D1 forces =
the
> grace period to be stalled on a task rather than a CPU, exercising a diff=
erent
> part of the RCU CPU stall warning code.
>=20
> In a CONFIG_PREEMPTION=3Dn kernel, using rcutorture.stall_cpu_block=3D1
> 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.
>=20

Verified.

If rcutorture.stall_cpu_block=3D1:
    There were lockdep and scheduling-while-atomic complaints for torture_t=
ype=3Drcu.
    No lockdep and scheduling-while-atomic complaints for torture_type=3Dsr=
cu.

> So these are test-the-diagnostics parameters.  The mdelay() instead makes
> rcutorture.stall_cpu_block=3D1 do the same thing as does
> rcutorture.stall_cpu_block=3D0 for CONFIG_PREEMPTION=3Dn kernels, right?

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

-Qiuxu

> 							Thanx, Paul