Re: [lkp-robot] [rcutorture] 46e26223e3: WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_stats_print

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Joel Fernandes <joelaf@google.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	kernel test robot <xiaolong.ye@intel.com>,
	linux-kernel@vger.kernel.org, kernel-team@android.com,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	mingo@redhat.com, Steven Rostedt <rostedt@goodmis.org>,
	tglx@linutronix.de, lkp@01.org
Subject: Re: [lkp-robot] [rcutorture]  46e26223e3: WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_stats_print
Date: Mon, 18 Jun 2018 19:38:59 -0700	[thread overview]
Message-ID: <20180619023859.GE3593@linux.vnet.ibm.com> (raw)
In-Reply-To: <2CDE3A5B-1AD3-4185-9E24-24D4075A9EA6@google.com>

On Mon, Jun 18, 2018 at 06:36:06PM -0700, Joel Fernandes wrote:
> 
> 
> On June 18, 2018 6:08:03 PM PDT, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >On Mon, Jun 18, 2018 at 03:26:47PM -0700, Joel Fernandes wrote:
> >> On Mon, Jun 18, 2018 at 09:56:46AM -0700, Paul E. McKenney wrote:
> >> > > The reason for the rcutorture test failure could be that the
> >default
> >> > > kthread_prio for the system's RCU threads is set to 1 (unless
> >overridden by
> >> > > rcutree.kthread_prio) which is also equal to the priority of the
> >rcutorture's
> >> > > boost threads. Due to this the rcutorture test could starve the
> >RCU threads
> >> > > as well and defeat the boosting mechanism. I was able to solve a
> >similar
> >> > > issue by just passing rcutree.kthread_prio of 50 on the kernel
> >command line.
> >> > > 
> >> > > Paul, would it be ok if we changed the default kthread_prio to
> >something > 1
> >> > > so that rcutorture can test properly without needing to pass any
> >extra
> >> > > rcutree.* parameters?
> >> > > 
> >> > > so something like this in kernel/rcu/tree.c ?
> >> > > 
> >> > > static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 2 : 0;
> >> > 
> >> > Would it be possible to also condition this on rcutorture being
> >built
> >> > in?  Or are they doing modprobes for rcutorture?
> >> 
> >> They seem to be doing built-in rcutorture tests. But I believe the
> >same
> >> problem would occur even if you used modules? I believe the fact that
> >> rcutorture is a module or built-in wouldn't matter to the underlying
> >issue
> >> which is the RCU subsystems's threads are at too low of a priority
> >> (rcutree.kthread_prio = 1).
> >
> >Understood...
> >
> >> If you agree with changing the default priority, I have included a
> >patch
> >> below for rcu/dev.
> >
> >The problem is that without rcutorture, rcutree.kthread_prio=1 is a
> >legitimate choice, and changing the default globally could be breaking
> >someone.  So it would be far better to up the priority only during
> >known
> >rcutorture testing.
> 
> Oh I see what you're saying. I'll work on a patch along these lines
> then. Thanks!

Looking forward to seeing it!

							Thanx, Paul