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
