Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit

Paul E. McKenney
2023-03-01T22:16:37Z

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.

It should be interesting seeing what comes up out of the IoT space. ;-)

Thanx, Paul