Re: question about rcuc/X tasks

[Chris Friesen <chris.friesen@windriver.com>]

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?

Thanks,
Chris