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

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Message-ID: <20180613072948.GA142000@joelaf.mtv.corp.google.com>
Date:   Wed, 13 Jun 2018 00:29:48 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     kernel test robot <xiaolong.ye@...el.com>
Cc:     Joel Fernandes <joelaf@...gle.com>, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        mingo@...hat.com, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>, tglx@...utronix.de,
        lkp@...org
Subject: Re: [lkp-robot] [rcutorture]  46e26223e3:
 WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_stats_print

(I'm actually not working this week, but still thought of replying :))

On Wed, Jun 13, 2018 at 02:57:11PM +0800, kernel test robot wrote:
> 
> FYI, we noticed the following commit (built with gcc-4.9):
> 
> commit: 46e26223e39c64763e321f229e324be15179c505 ("rcutorture: Make boost test more robust")
> url: https://github.com/0day-ci/linux/commits/Joel-Fernandes/rcutorture-Disable-RT-throttling-for-boost-tests/20180611-074731
> base: https://git.kernel.org/cgit/linux/kernel/git/paulmck/linux-rcu.git rcu/next
> 
> in testcase: trinity
> with following parameters:
> 
> 	runtime: 300s
> 
> test-description: Trinity is a linux system call fuzz tester.
> test-url: http://codemonkey.org.uk/projects/trinity/

I'll try to reproduce this, but it could be because after this patch, the
boost test is actually working.

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;

thanks!

 - Joel