linux-kernel - Re: [PATCH v2 3/6] rcu: limit PREEMPT

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Message-ID: <ZzTGQ_zTS8NrxjW9@localhost.localdomain>
Date: Wed, 13 Nov 2024 16:31:15 +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 v2 3/6] rcu: limit PREEMPT_RCU configurations

Le Wed, Nov 06, 2024 at 12:17:55PM -0800, Ankur Arora a écrit :
> PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNAMIC
> which allows for dynamic switching of preemption models.
> 
> The choice of PREEMPT_RCU or not, however, is fixed at compile time.
> 
> Given that PREEMPT_RCU makes some trade-offs to optimize for latency
> as opposed to throughput, configurations with limited preemption
> might prefer the stronger forward-progress guarantees of PREEMPT_RCU=n.
> 
> Accordingly, explicitly limit PREEMPT_RCU=y to the latency oriented
> preemption models: PREEMPT, PREEMPT_RT, and the runtime configurable
> model PREEMPT_DYNAMIC.
> 
> This means the throughput oriented models, PREEMPT_NONE,
> PREEMPT_VOLUNTARY and PREEMPT_LAZY will run with PREEMPT_RCU=n.
> 
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  kernel/rcu/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 5a7ff5e1cdcb..9d52f87fac27 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -18,7 +18,7 @@ config TREE_RCU
>  
>  config PREEMPT_RCU
>  	bool
> -	default y if PREEMPTION
> +	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
>  	select TREE_RCU
>  	help
>  	  This option selects the RCU implementation that is

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

But looking at !CONFIG_PREEMPT_RCU code on tree_plugin.h, I see
some issues now that the code can be preemptible. Well I think
it has always been preemptible but PREEMPTION && !PREEMPT_RCU
has seldom been exerciced (or was it even possible?).

For example rcu_read_unlock_strict() can be called with preemption
enabled so we need the following otherwise the rdp is unstable, the
norm value becomes racy (though automagically fixed in rcu_report_qs_rdp())
and rcu_report_qs_rdp() might warn.

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 58d84c59f3dd..368f00267d4e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -95,9 +95,9 @@ static inline void __rcu_read_lock(void)
 
 static inline void __rcu_read_unlock(void)
 {
-	preempt_enable();
 	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
 		rcu_read_unlock_strict();
+	preempt_enable();
 }
 
 static inline int rcu_preempt_depth(void)


Let me audit further if we missed something else...

Thanks.

> -- 
> 2.43.5
>