On 12/15/2016 08:07 AM, Steven Rostedt wrote:

On Thu, 15 Dec 2016 14:47:37 +0100
Daniel Bristot de Oliveira [off-list ref] wrote:

quoted

Hi Chris,

On 12/12/2016 11:42 PM, Chris Friesen wrote:

quoted

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.

Paul, would rcu_nocbs=1-15 work? Or should ALL be used ? I'm assuming
this is on a 16 CPUs box, in which case I don't see much of a difference
for not just using ALL as it is almost there anyway ;-)

-- Steve

Yes, this was a 16 CPU box.

The blocker for CONFIG_RCU_NOCB_CPU_ALL is that the set of 
management/housekeeping CPUs is configurable by the end-user, so while it 
defaults to only CPU0 as management it's not guaranteed that it will always be 
that way.

On a related note, I found an old email from Paul suggesting that the various 
rcuc/X threads could be affined to the management CPUs to free up the "realtime" 
cores, but when I try that it doesn't let me change affinity.  Was that 
disallowed for technical reasons?  (It's also possible it's something local, in 
which case I need to go digging.)

Thanks,
Chris