linux-kernel - Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt

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Date:   Wed, 1 Mar 2023 14:25:36 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Usama Arif <usama.arif@...edance.com>, dwmw2@...radead.org,
        kim.phillips@....com, brgerst@...il.com, piotrgorski@...hyos.org,
        oleksandr@...alenko.name, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, rcu@...r.kernel.org, mimoja@...oja.de,
        hewenliang4@...wei.com, thomas.lendacky@....com, seanjc@...gle.com,
        pmenzel@...gen.mpg.de, fam.zheng@...edance.com,
        punit.agrawal@...edance.com, simon.evans@...edance.com,
        liangma@...ngbit.com, David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit

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".)

- Josh