Re: question about rcuc/X tasks

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

On Thu, Dec 15, 2016 at 09:20:24AM -0600, Chris Friesen wrote:
> On 12/15/2016 08:07 AM, 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 ;-)
> >
> >-- 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.)

The rcuo/X kthreads can be affined, but the rcuc/X kthreads must run on
the corresponding CPU for correctness reasons -- they communicate with
RCU core using protocols that are only single-CPU-safe.  But if you are
running NO_HZ_FULL, these kthreads should never run unless your user
threads are doing syscalls.

So, are they actually running in your setup?

							Thanx, Paul