Re: question about rcuc/X tasks

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

On Thu, Dec 15, 2016 at 04:23:27PM -0600, Chris Friesen wrote:
> On 12/15/2016 01:04 PM, Paul E. McKenney wrote:
> >On Thu, Dec 15, 2016 at 09:20:24AM -0600, Chris Friesen wrote:
> 
> >>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?
> 
> Yes, but I wasn't setting nohz_full.  With "rcu_nocb_poll
> isolcpus=1-15 rcu_nocbs=1-15 nohz_full=1-15" I'm not seeing the
> rcuc/X kthreads running.
> 
> So in the non-nohz_full case, what are they waking up to do?
> Something timer-related?

Interesting.  I need to look into this a bit.  I would not expect
that rcuc/X kthreads corresponding to NOCB CPUs to ever wake up.
(They are created by a per-CPU facility that creates a kthread per
CPU no matter what.)

							Thanx, Paul