AW: RCU simplification and RT needs

[eg Engleder Gerhard <eg@keba.com>]

> Hello!
Hello Paul,
> 
> At Linus's request, I am simplifying the Linux-kernel RCU implementation, which
> includes removing code that implements features and options that are no longer
> needed.  This is not a half-hearted effort.  In fact, I expect that my submission to
> the next merge window will be a net removal of more than 2500 lines of code.
> 
> But wait, there is more!  ;-)
> 
> Although the following two features are not being axed in v4.13, they will be in
> v4.14 unless someone makes a convincing case for them:
> 
> 1.	The ability to build a CONFIG_RCU_NOCB_CPUS=y kernel without
> 	also specifying CONFIG_NO_HZ_FULL.
> 
> 	Unless someone speaks for this configuration option,
> 	CONFIG_RCU_NOCB_CPUS will be slaved off of CONFIG_NO_HZ_FULL,
> 	and the rcu_nocbs= boot parameter will be dropped.  (RCU would
> 	instead use the nohz_full= boot parameter to determine which
> 	CPUs get their callbacks offloaded.)

We are using CONFIG_RCU_NOCB_CPUS=y without CONFIG_NO_HZ_FULL
with 4.9-rt in our products.
We are not using CONFIG_NO_HZ_FULL, because its use was not recommended for
real time. We set CONFIG_RCU_NOCB_CPUS=y and CONFIG_RCU_NOCB_CPU_ALL=y
to reduce the jitter. This configuration showed the minimal jitter with our application.

We are not relying on the configuration itself. Our goal is just a minimum jitter. Thus, if
CONFIG_NO_HZ_FULL can be used for real time with no negative impact on the
jitter, then we can enable this option.
@tglx: Is CONFIG_NO_HZ_FULL the preferred choice for real-time kernels?

Gerhard