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Date:   Wed, 1 Mar 2023 17:05:39 -0800
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Subject: Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit
To:     paulmck@kernel.org, Josh Triplett <josh@joshtriplett.org>
Cc:     Arjan van de Ven <arjan@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Usama Arif <usama.arif@bytedance.com>, dwmw2@infradead.org,
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From:   Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20230302002851.GK2948950@paulmck-ThinkPad-P17-Gen-1>
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On 3/1/23 16:28, Paul E. McKenney wrote:
> On Wed, Mar 01, 2023 at 02:25:36PM -0800, Josh Triplett wrote:
>> On Wed, Mar 01, 2023 at 02:16:32PM -0800, Paul E. McKenney wrote:
>>> On Wed, Mar 01, 2023 at 12:32:26PM -0800, Josh Triplett wrote:
>>>> On Tue, Feb 28, 2023 at 01:02:33PM -0800, Arjan van de Ven wrote:
>>>>   Thomas Gleixner wrote:
>>>>>>
>>>>>> Maybe we should enforce CONFIG_SMP=y first :)
>>>>>>
>>>>>> Thanks,
>>>>>
>>>>> for 64 bit I can see the point of removing the !SMP case entirely from arch/x86 .
>>>>> maybe even for 32 bit if it just makes the code simpler I suppose
>>>>
>>>> As one of the folks keeping an eye on tinyconfig and kernel size, I
>>>> actually think we *should* make this change and rip out !CONFIG_SMP,
>>>> albeit carefully.
>>>>
>>>> In particular, I would propose that we rip out !CONFIG_SMP, *but* we
>>>> allow building with CONFIG_NR_CPUS=1. (And we could make sure in that
>>>> case that the compiler can recognize that at compile time and optimize
>>>> accordingly, so that it might provide some of the UP optimizations for
>>>> us.)
>>>>
>>>> Then, any *optimizations* for the "will only have one CPU, ever" case
>>>> can move to CONFIG_NR_CPUS=1 rather than !CONFIG_SMP. I think many of
>>>> those optimizations may be worth keeping for small embedded systems, or
>>>> for cases like Linux-as-bootloader or similar.
>>>>
>>>> The difference here would be that code written for !CONFIG_SMP today
>>>> needs to account for the UP case for *correctness*, whereas code written
>>>> for CONFIG_SMP can *optionally* consider CONFIG_NR_CPUS=1 for
>>>> *performance*.
>>>
>>> It certainly would not make much sense to keep Tiny RCU and Tiny SRCU
>>> around if there was no CONFIG_SMP=n.
>>
>> On the contrary, I think it's entirely appropriate to keep them for
>> CONFIG_NR_CPUS=1; that's exactly the kind of simple optimization that
>> seems well worth having. (Ideal optimization: "very very simple for UP,
>> complex for SMP"; non-ideal optimization: "complex for SMP, differently
>> complex for UP".)
> 
> Fair enough, but how does removing CONFIG_SMP help with that?  Given that
> it is not all that hard to work around the lack of CONFIG_SMP for Tiny
> RCU and Tiny SRCU, then it cannot be all that hard to work around that
> lack for the use cases that you are trying to get rid of, right?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 9071182b1284b..7487bee3d4341 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -7,7 +7,7 @@ menu "RCU Subsystem"
>  
>  config TREE_RCU
>  	bool
> -	default y if SMP
> +	default y if CONFIG_NR_CPUS = 1
>  	# Dynticks-idle tracking
>  	select CONTEXT_TRACKING_IDLE
>  	help
> @@ -31,7 +31,7 @@ config PREEMPT_RCU
>  
>  config TINY_RCU
>  	bool
> -	default y if !PREEMPTION && !SMP
> +	default y if !PREEMPTION && CONFIG_NR_CPUS != 1
>  	help
>  	  This option selects the RCU implementation that is
>  	  designed for UP systems from which real-time response

but drop the CONFIG_ prefixes...

-- 
~Randy