question about rcuc/X tasks

[Chris Friesen <chris.friesen@windriver.com>]

Hi,

I'm testing the CentOS 7.3 RT kernel, 3.10.0-514.rt56.420

(I realize this is a heavily-patched kernel, but I'm hoping general information 
will still apply.)

My kernel is booted with "isolcpus=1-15 rcu_nocbs=1-15" as kernel parameters, 
and /dev/cpu_dma_latency is set to 3.  The relevent config options are set as 
follows:

CONFIG_PREEMPT=y
CONFIG_PREEMPT_RT_BASE=y
CONFIG_HAVE_PREEMPT_LAZY=y
CONFIG_PREEMPT_LAZY=y
CONFIG_PREEMPT_RT_FULL=y
CONFIG_RCU_BOOST=y
CONFIG_RCU_KTHREAD_PRIO=2
CONFIG_RCU_BOOST_DELAY=500
CONFIG_RCU_NOCB_CPU=y
CONFIG_RCU_NOCB_CPU_NONE=y

The issue I'm seeing is that when I run cyclictest, ftrace shows rcuc/<cpu> 
running for 5-6 usec quite a few times per second.

Based on the fact that Documentation/kernel-per-CPU-kthreads.txt describes 
CONFIG_RCU_NOCB_CPU_ALL=y as a solution by preventing the rcuc/%u kthreads from 
having any work to do, I had expected that the "rcu_nocbs=1-15" kernel parameter 
would have a similar effect.

Does anyone have any idea what's going on here, and how I can prevent the 
rcuc/<cpu> tasks from waking up?

Thanks,
Chris