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

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Message-ID: <87bjyir7dk.fsf@oracle.com>
Date: Wed, 13 Nov 2024 16:23:03 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Frederic Weisbecker <frederic@...nel.org>
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


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

> 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>

Thanks!

> 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

I think I see your point about rdp being racy, but given that
rcu_read_unlock_strict() would always be called with a non-zero
preemption count (with CONFIG_PREEMPTION), wouldn't the preempt_count()
check defeat any calls to rcu_read_unlock_strict()?

    void rcu_read_unlock_strict(void)
    {
            struct rcu_data *rdp;

            if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
                    return;

Or am I missing something?

Ankur

> (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
>>