Date: Fri, 6 Jul 2018 08:52:00 -0700 (PDT) From: "Rodney W. Grimes" <freebsd@pdx.rh.CN85.dnsmgr.net> To: Warner Losh <imp@bsdimp.com> Cc: "Rodney W. Grimes" <rgrimes@freebsd.org>, Hans Petter Selasky <hselasky@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r336025 - in head/sys: amd64/include i386/include Message-ID: <201807061552.w66Fq0FX052931@pdx.rh.CN85.dnsmgr.net> In-Reply-To: <CANCZdfpZpeCVSs1%2BMptwuN76SED-W4XyLCyQfrd4OpGwFk8Hrg@mail.gmail.com>
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> On Fri, Jul 6, 2018 at 9:32 AM, Rodney W. Grimes < > freebsd@pdx.rh.cn85.dnsmgr.net> wrote: > > > > Author: hselasky > > > Date: Fri Jul 6 10:13:42 2018 > > > New Revision: 336025 > > > URL: https://svnweb.freebsd.org/changeset/base/336025 > > > > > > Log: > > > Make sure kernel modules built by default are portable between UP and > > > SMP systems by extending defined(SMP) to include defined(KLD_MODULE). > > > > > > This is a regression issue after r335873 . > > > > > > Discussed with: mmacy@ > > > Sponsored by: Mellanox Technologies > > > > Though this fixes the issue, it also means that now when > > anyone intentionally builds a UP kernel with modules > > they are getting SMP support in the modules and I am > > not sure they would want that. I know I don't. > > > > > On UP systems, these additional opcodes are harmless. They take a few extra > cycles (since they lock an uncontested bus) and add a couple extra memory > barriers (which will be NOPs). On MP systems, atomics now work by default. > Had we not defaulted like this, all modules built outside of a kernel build > env would have broken atomics. Given that (a) the overwhelming majority > (99% or more) is SMP and (b) the MP code merely adds a few cycles to what's > already a not-too-expensive operation, this was the right choice. > > It simply doesn't matter for systems that are relevant to the project > today. While one could try to optimize this a little (for example, by > having SMP defined to be 0 or 1, say, and changing all the ifdef SMP to if > (defined(SMP) && SMP != 0)), it's likely not going to matter enough for > anybody to make the effort. UP on x86 is simply not relevant enough to > optimize for it. Even in VMs, people run SMP kernels typically even when > they just allocate one CPU to the VM. > > So while we still support the UP config, and we'll let people build > optimized kernels for x86, we've flipped the switch from pessimized for SMP > modules to pessimized for UP modules, which seems like quite the reasonable > trade-off. > > Were it practical to do so, I'd suggest de-orbiting UP on x86. However, > it's a lot of work for not much benefit and we'd need to invent much crazy > to get there. Trivial to fix this with +#if defined(SMP) || !defined(_KERNEL) || defined(KLD_MODULE) || !defined(KLD_UP_MODULES) > > Warner > > > > > Modified: > > > head/sys/amd64/include/atomic.h > > > head/sys/i386/include/atomic.h > > > > > > Modified: head/sys/amd64/include/atomic.h > > > ============================================================ > > ================== > > > --- head/sys/amd64/include/atomic.h Fri Jul 6 10:10:00 2018 > > (r336024) > > > +++ head/sys/amd64/include/atomic.h Fri Jul 6 10:13:42 2018 > > (r336025) > > > @@ -132,7 +132,7 @@ void atomic_store_rel_##TYPE(volatile > > u_##TYPE *p, u_ > > > * For userland, always use lock prefixes so that the binaries will run > > > * on both SMP and !SMP systems. > > > */ > > > -#if defined(SMP) || !defined(_KERNEL) > > > +#if defined(SMP) || !defined(_KERNEL) || defined(KLD_MODULE) > > > #define MPLOCKED "lock ; " > > > #else > > > #define MPLOCKED > > > @@ -354,7 +354,7 @@ atomic_testandclear_long(volatile u_long *p, u_int > > v) > > > */ > > > #define OFFSETOF_MONITORBUF 0x100 > > > > > > -#if defined(SMP) > > > +#if defined(SMP) || defined(KLD_MODULE) > > > static __inline void > > > __storeload_barrier(void) > > > { > > > > > > Modified: head/sys/i386/include/atomic.h > > > ============================================================ > > ================== > > > --- head/sys/i386/include/atomic.h Fri Jul 6 10:10:00 2018 > > (r336024) > > > +++ head/sys/i386/include/atomic.h Fri Jul 6 10:13:42 2018 > > (r336025) > > > @@ -143,7 +143,7 @@ void atomic_subtract_64(volatile > > uint64_t *, uint64_t > > > * For userland, always use lock prefixes so that the binaries will run > > > * on both SMP and !SMP systems. > > > */ > > > -#if defined(SMP) || !defined(_KERNEL) > > > +#if defined(SMP) || !defined(_KERNEL) || defined(KLD_MODULE) > > > #define MPLOCKED "lock ; " > > > #else > > > #define MPLOCKED > > > @@ -302,7 +302,7 @@ atomic_testandclear_int(volatile u_int *p, u_int v) > > > */ > > > > > > #if defined(_KERNEL) > > > -#if defined(SMP) > > > +#if defined(SMP) || defined(KLD_MODULE) > > > #define __storeload_barrier() __mbk() > > > #else /* _KERNEL && UP */ > > > #define __storeload_barrier() __compiler_membar() > > > > > > > > > > -- > > Rod Grimes > > rgrimes@freebsd.org > > > > -- Rod Grimes rgrimes@freebsd.org
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