Re: [PATCH v2 5/6] osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y

[Ankur Arora <ankur.a.arora@oracle.com>]

Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> On 2024-11-06 12:17:57 [-0800], Ankur Arora wrote:
>> To reduce RCU noise for nohz_full configurations, osnoise depends
>> on cond_resched() providing quiescent states for PREEMPT_RCU=n
>> configurations. And, for PREEMPT_RCU=y configurations does this
>> by directly calling rcu_momentary_eqs().
>>
>> With PREEMPT_LAZY=y, however, we can have configurations with
>> (PREEMPTION=y, PREEMPT_RCU=n), which means neither of the above
>> can help.
>
> The problem is as you say CONFIG_PREEMPT_RCU=n + CONFIG_PREEMPTION=y.
> You can't select any of those two directly but get here via
>  PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n.
>
> Please spell it out to make it obvious. It is not a large group of
> configurations, it is exactly this combo.

Makes sense.

Will do. Thanks.

Ankur

>  With PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n however we get PREEMPT_RCU=n
>  which means no direct rcu_momentary_eqs() invocations and
>  cond_resched() is an empty stub.
>
>> Handle that by fallback to the explicit quiescent states via
>> rcu_momentary_eqs().
>
>> Cc: Paul E. McKenney <paulmck@kernel.org>
>> Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
>> Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>
> Sebastian


--
ankur