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Message-ID: <Z0h_KMzCCx2umo6h@localhost.localdomain>
Date: Thu, 28 Nov 2024 15:33:12 +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,
	Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [PATCH v2 5/6] osnoise: handle quiescent states for
 PREEMPT_RCU=n, PREEMPTION=y

Le Wed, Nov 06, 2024 at 12:17:57PM -0800, Ankur Arora a écrit :
> 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.
> 
> 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>
> ---
>  kernel/trace/trace_osnoise.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
> index a50ed23bee77..15e9600d231d 100644
> --- a/kernel/trace/trace_osnoise.c
> +++ b/kernel/trace/trace_osnoise.c
> @@ -1538,18 +1538,20 @@ static int run_osnoise(void)
>  		/*
>  		 * In some cases, notably when running on a nohz_full CPU with
>  		 * a stopped tick PREEMPT_RCU has no way to account for QSs.
> -		 * This will eventually cause unwarranted noise as PREEMPT_RCU
> -		 * will force preemption as the means of ending the current
> -		 * grace period. We avoid this problem by calling
> -		 * rcu_momentary_eqs(), which performs a zero duration
> -		 * EQS allowing PREEMPT_RCU to end the current grace period.
> -		 * This call shouldn't be wrapped inside an RCU critical
> -		 * section.
> +		 * This will eventually cause unwarranted noise as RCU forces
> +		 * preemption as the means of ending the current grace period.
> +		 * We avoid this by calling rcu_momentary_eqs(), which performs
> +		 * a zero duration EQS allowing RCU to end the current grace
> +		 * period. This call shouldn't be wrapped inside an RCU
> +		 * critical section.
>  		 *
> -		 * Note that in non PREEMPT_RCU kernels QSs are handled through
> -		 * cond_resched()
> +		 * For non-PREEMPT_RCU kernels with cond_resched() (non-
> +		 * PREEMPT_LAZY configurations), QSs are handled through
> +		 * cond_resched(). For PREEMPT_LAZY kernels, we fallback to
> +		 * the zero duration QS via rcu_momentary_eqs().
>  		 */
> -		if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
> +		if (IS_ENABLED(CONFIG_PREEMPT_RCU) ||
> +		    (!IS_ENABLED(CONFIG_PREEMPT_RCU) && IS_ENABLED(CONFIG_PREEMPTION))) {
>  			if (!disable_irq)
>  				local_irq_disable();

How about making this unconditional so it works everywhere and doesn't
rely on cond_resched() Kconfig/preempt-dynamic mood?

Thanks.


>  
> -- 
> 2.43.5
> 

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