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

[Ankur Arora]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> 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.

Agreed.

> 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.

Also, agreed.

> 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, RCU can deal with (PREEMPT_RCU=y, PREEMPT_AUTO=y, preempt=none/voluntary/full).
Since that's basically what PREEMPT_DYNAMIC already works with.

The other combination, (PREEMPT_RCU=n, PREEMPT_AUTO,
preempt=none/voluntary) would generally be business as usual, except, as
you say, it is really PREEMPT_RCU=n, preempt=full in disguise.

However, as Paul says __rcu_read_lock(), for PREEMPT_RCU=n is defined as:

static inline void __rcu_read_lock(void)
{
        preempt_disable();
}

So, this combination -- though non standard -- should also work.

The reason for adding the warning was because Paul had warned in
discussions earlier (see here for instance:
https://lore.kernel.org/lkml/842f589e-5ea3-4c2b-9376-d718c14fabf5@paulmck-laptop/)

that the PREEMPT_FULL=y and PREEMPT_RCU=n is basically useless. But at
least in my understanding that's primarily a performance concern not a
correctness concern. But, Paul can probably speak to that more.

  "PREEMPT_FULL=y plus PREEMPT_RCU=n appears to be a useless
  combination.  All of the gains from PREEMPT_FULL=y are more than lost
  due to PREEMPT_RCU=n, especially when the kernel decides to do something
  like walk a long task list under RCU protection.  We should not waste
  people's time getting burned by this combination, nor should we waste
  cycles testing it."


--
ankur