linux-kernel - Re: [PATCH v2 4/6] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT

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Message-ID: <87wmh5nlek.fsf@oracle.com>
Date: Thu, 14 Nov 2024 20:58:11 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: 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
Subject: Re: [PATCH v2 4/6] rcu: handle quiescent states for PREEMPT_RCU=n,
 PREEMPT_COUNT=y


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

> On 2024-11-06 12:17:56 [-0800], Ankur Arora wrote:
>> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
>> states for read-side critical sections via rcu_all_qs().
>> One reason why this was needed, was lacking preempt-count, the tick
>> handler has no way of knowing whether it is executing in a read-side
>> critical section or not.
>>
>> With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
>> PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
>> quiescent states via rcu_all_qs().
>
> With PREEMPT_LAZY=y && PREEMPT_DYNAMIC=n we get PREEMPT_COUNT=y and
> PREEMPT_RCU=n. In this configuration cond_resched() is an empty stub and
> does not provide quiescent states via rcu_all_qs(). PREEMPT_RCU=y
> provides this information via rcu_read_unlock() and its nesting counter.
>
>> So, use the availability of preempt_count() to report quiescent states
>> in rcu_flavor_sched_clock_irq().
>
> Okay. You might also want to update the cond_resched() comment,
> 	s@In preemptible kernels, ->rcu_read_lock_nesting@
> 	  In PREEMPT_RCU kernels, ->rcu_read_lock_nesting@

Good point. Will add.

> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Thanks!

Ankur

>> Suggested-by: Paul E. McKenney <paulmck@...nel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>
> Sebastian


--
ankur