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

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Message-ID: <Z1w9ydTN90Sl8qda@localhost.localdomain>
Date: Fri, 13 Dec 2024 14:59:37 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: 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

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>