linux-kernel - Re: [PATCH v2 5/6] osnoise: handle quiescent states for PREEMPT

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Message-ID: <87sertnlbv.fsf@oracle.com>
Date: Thu, 14 Nov 2024 20:59:48 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
        mingo@...nel.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, vschneid@...hat.com, frederic@...nel.org,
        efault@....de, sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
        Daniel Bristot de
 Oliveira <bristot@...nel.org>
Subject: Re: [PATCH v2 5/6] osnoise: handle quiescent states for
 PREEMPT_RCU=n, PREEMPTION=y


Sebastian Andrzej Siewior <bigeasy@...utronix.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@...nel.org>
>> Cc: Daniel Bristot de Oliveira <bristot@...nel.org>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Suggested-by: Paul E. McKenney <paulmck@...nel.org>
>> Acked-by: Daniel Bristot de Oliveira <bristot@...nel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>
> Sebastian


--
ankur