Re: [RFC PATCH 47/86] rcu: select PREEMPT_RCU if PREEMPT

[Steven Rostedt]

On Mon, 20 Nov 2023 21:04:28 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> How about like this, where "Y" means allowed and "N" means not allowed:
> 
> 			Non-Preemptible RCU	Preemptible RCU
> 
> NONE:				Y			Y
> 
> VOLUNTARY:			Y			Y
> 
> PREEMPT:			N			Y
> 
> PREEMPT_RT:			N			Y
> 
> 
> We need preemptible RCU for NONE and VOLUNTARY, as you say,
> to allow CONFIG_PREEMPT_DYNAMIC to continue to work.  (OK, OK,
> CONFIG_PREEMPT_DYNAMIC is no longer, but appears to be unconditional.)
> But again, I don't see why anyone would want (much less need)
> non-preemptible RCU in the PREEMPT and PREEMPT_RT cases.  And if it is
> neither wanted nor needed, there is no point in enabling it, much less
> testing it.
> 
> Or am I missing a use case in there somewhere?

As Ankur replied, this is just an RFC, not the main goal. I'm talking about
the end product which will get rid of the PREEMPT_NONE, PREEMPT_VOLUNTARY
and PREEMPT conifgs, and there will *only* be the PREEMPT_DYNAMIC and
PREEMPT_RT.

And yes, this is going to be a slow and long processes, to find and fix all
regressions. I too am concerned about the latency that this may add. I'm
thinking we could have NEED_RESCHED_LAZY preempt when there is no mutex or
other semi critical section held (like migrate_disable()).

Right now, the use of cond_resched() is basically a whack-a-mole game where
we need to whack all the mole loops with the cond_resched() hammer. As
Thomas said, this is backwards. It makes more sense to just not preempt in
areas that can cause pain (like holding a mutex or in an RCU critical
section), but still have the general kernel be fully preemptable.

-- Steve