Re: question about rcuc/X tasks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Dec 15, 2016 at 09:07:14AM -0500, Steven Rostedt wrote:
> 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 ;-)

Using ALL is equivalent to rcu_nocbs=0-15 (as opposed to rcu_nocbs=1-15) on
a 16-CPU box, assuming that the CPUs are numbered consecutively.  The only
case where I could see a difference is if you were running something like
Rik's nasty Java workload on CPU 0, in which case rcu_nocbs=1-15 would be
the better choice.

							Thanx, Paul

> -- 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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