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
