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SubjectRe: Real-Time Preemption and RCU

* Paul E. McKenney <paulmck@us.ibm.com> wrote:

> For the patch, here are my questions:
>
> o What is the best way to select between classic RCU and this
> scheme?
>
> 1. Massive #ifdef across rcupdate.c
>
> 2. Create an rcupdate_rt.c and browbeat the build system
> into picking one or the other (no clue if this is
> possible...)
>
> 3. Create an rcupdate_rt.c and rely on the linker to pick
> one or the other, with rcupdate.h generating different
> external symbol names to make the choice.

you can also go for option #0: just replace the existing RCU code with
the new one, and i'll then deal with the configuration details.

what will have to happen is most likely #2 (since there is near zero
code sharing between the two variants, right?). Picking rcupdate_rt.c is
as simple as doing this:

obj-$(CONFIG_DONT_PREEMPT_RCU) += rcupdate.o
obj-$(CONFIG_PREEMPT_RCU) += rcupdate_rt.o

and then use Kconfig to generate either CONFIG_DONT_PREEMPT_RCU
(default) or CONFIG_PREEMPT_RCU (if the user selects it).

but it's not yet clear whether we want to offer this to users as a
configurable option. The simplest solution for you would be to go with
option #0 :-) [or if you prefer switchability, #1 is good too - i can
then extract the bits and do #2 based on that.]

> o How best to interface to OOM? Left to myself, I leave this
> for later. ;-)

yeah, i'd not worry about OOM that much at this stage.

> I will take the cowardly approach of patching against the upstream
> kernel.

sure. This is in fact easier for me: i'll first rip all my RCU hackery
out of -RT and then add your code, so the base i'll be merging against
will be closer to upstream than to current -RT.

Ingo
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