linux-kernel - Re: [PATCH v3 5/7] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT

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Message-ID: <874j379uu1.fsf@oracle.com>
Date: Fri, 13 Dec 2024 12:44:22 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Frederic Weisbecker <frederic@...nel.org>
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, bigeasy@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, efault@....de, sshegde@...ux.ibm.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v3 5/7] rcu: handle quiescent states for PREEMPT_RCU=n,
 PREEMPT_COUNT=y


Frederic Weisbecker <frederic@...nel.org> writes:

> Le Thu, Dec 12, 2024 at 08:06:56PM -0800, Ankur Arora a écrit :
>> 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: 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, PREEMPT_DYNAMIC=n), we get (PREEMPT_COUNT=y,
>> PREEMPT_RCU=n). In this configuration cond_resched() is a 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().
>>
>> Suggested-by: Paul E. McKenney <paulmck@...nel.org>
>> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks for all the reviews!

--
ankur