Re: [PATCH v2 16/35] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 27, 2024 at 05:35:02PM -0700, Ankur Arora wrote:
> The combination of PREEMPT_RCU=n and (PREEMPT_AUTO=y, preempt=full)
> works at cross purposes: the RCU read side critical sections disable
> preemption, while preempt=full schedules eagerly to minimize
> latency.
> 
> Warn if the user is switching to full preemption with PREEMPT_RCU=n.
> 
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Link: https://lore.kernel.org/lkml/842f589e-5ea3-4c2b-9376-d718c14fabf5@paulmck-laptop/
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>  kernel/sched/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d7804e29182d..df8e333f2d8b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8943,6 +8943,10 @@ static void __sched_dynamic_update(int mode)
>  		break;
>  
>  	case preempt_dynamic_full:
> +		if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
> +			pr_warn("%s: preempt=full is not recommended with CONFIG_PREEMPT_RCU=n",
> +				PREEMPT_MODE);
> +

Yeah, so I don't believe this is a viable strategy.

Firstly, none of these RCU patches are actually about the whole LAZY
preempt scheme, they apply equally well (arguably better) to the
existing PREEMPT_DYNAMIC thing.

Secondly, esp. with the LAZY thing, you are effectively running FULL at
all times. It's just that some of the preemptions, typically those of
the normal scheduling class are somewhat delayed. However RT/DL classes
are still insta preempt.

Meaning that if you run anything in the realtime classes you're running
a fully preemptible kernel. As such, RCU had better be able to deal with
it.

So no, I don't believe this is right.