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

[Paul E. McKenney]

On Thu, May 30, 2024 at 04:05:26PM -0700, Ankur Arora wrote:
> 
> Paul E. McKenney <paulmck@xxxxxxxxxx> writes:
> 
> > On Wed, May 29, 2024 at 10:14:04AM +0200, Peter Zijlstra wrote:
> >> 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@xxxxxxxxxx>
> >> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> > Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> >> > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> >> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> >> > Link: https://lore.kernel.org/lkml/842f589e-5ea3-4c2b-9376-d718c14fabf5@paulmck-laptop/
> >> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> >> > ---
> >> >  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.
> >
> > At one point, lazy preemption selected PREEMPT_COUNT (which I am
> > not seeing in this version, perhaps due to blindness on my part).
> > Of course, selecting PREEMPT_COUNT would result in !PREEMPT_RCU kernel's
> > rcu_read_lock() explicitly disabling preemption, thus avoiding preemption
> > (including lazy preemption) in RCU read-side critical sections.
> 
> That should be still happening, just transitively. PREEMPT_AUTO selects
> PREEMPT_BUILD, which selects PREEMPTION, and that in turn selects
> PREEMPT_COUNT.

Ah, I gave up too soon.  Thank you!

							Thanx, Paul