Re: question about rcuc/X tasks

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 15 Dec 2016 14:47:37 +0100
Daniel Bristot de Oliveira <daniel@bristot.me> wrote:

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

> 
> As far as I recall, using CONFIG_RCU_NOCB_CPU_ALL=y will prevent the RCU
> callbacks from running on rcuc/X threads (which are pinned to the CPU
> X), but the rcuc/X thread is still in charge of awaking the ruco*
> threads of the CPU X.
> 
> > Does anyone have any idea what's going on here, and how I can prevent
> > the rcuc/<cpu> tasks from waking up?  
> 
> You can try to use rcu_nocb_poll kernel parameter to unload the rcuc/X
> thread of the task of awaking its respective ruco*s threads.
> 
> >From [1]:  
> 
> rcu_nocb_poll	[KNL]
> 			Rather than requiring that offloaded CPUs
> 			(specified by rcu_nocbs= above) explicitly
> 			awaken the corresponding "rcuoN" kthreads,
> 			make these kthreads poll for callbacks.
> 			This improves the real-time response for the
> 			offloaded CPUs by relieving them of the need to
> 			wake up the corresponding kthread, but degrades
> 			energy efficiency by requiring that the kthreads
> 			periodically wake up to do the polling.
> 
> [1] https://www.kernel.org/doc/Documentation/kernel-parameters.txt
> 
> -- Daniel
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