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

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Message-ID: <20241114085027.cH9ZY0OD@linutronix.de>
Date: Thu, 14 Nov 2024 09:50:27 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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, 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

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@

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

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

Sebastian