On Mon, Feb 02, 2015 at 01:39:33PM +0000, Will Deacon wrote: > On Fri, Jan 30, 2015 at 08:44:48AM +0000, Yong Zhang wrote: > > On Fri, Jan 30, 2015 at 2:56 AM, Nathan Sullivan <nathan.sullivan@xxxxxx<mailto:nathan.sullivan@xxxxxx>> wrote: > > > > While investigating a performance issue on RT 3.14, I noticed that > > __HAVE_ARCH_CMPXCHG is not defined for ARM. This causes -rt locks to > > always use the slow path and impacts performance. ARMv6 and above > > have a cmpxchg implementation already that will work just fine. > > > > In 2012, Yong Zhang submitted a patch to fix this[1], but it mysteriously > > disappeared afterwards. I did notice that the s-o-b line has a different > > email address than the windriver one. Yong, do you mind re-submitting the > > > > It actually doesn't matter. Anyway feel free to add another s-o-b to > > the patch: > > > > Signed-off-by: Yong Zhang <yong.zhang@xxxxxxxxxxxxx<mailto:yong.zhang@xxxxxxxxxxxxx>> I did a quick scan of architectures and looked at the various cmpxchg implementations. Assuming that __HAVE_ARCH_CMPXCHG really means "this architecture implements a native cmpxchg which is faster than the generic implementation", I put what I found in the following table; note that I'm not 100% confident these are correct, as in some instances I had to make a judgement call as to whether the implementation is "fast", hopefully it's still useful :). native native cmpxchg (UP) cmpxchg (SMP) __HAVE_ARCH_CMPXCHG alpha X X X arc X[1] X[1] arm (pre ARMv6) [2] arm (ARMv6+) X X arm64 X X avr32 X X X blackfin X c6x [2] cris [3] frv X X hexagon X X X ia64 X X X m32r X X X m68k X[4] X[4] X[4] metag X X X microblaze [2] mips X X mn10300 X[5] nios2 [2] openrisc [6] parisc X X X powerpc X X X s390 X X X score [7] [7] X[7] sh X X X sparc X X X tile X X unicore32 [2] x86 X X X xtensa X X 1: Conditional on CONFIG_ARC_HAS_LLSC 2: SMP not a supported configuration 3: No cmpxchg() implementation at all? 4: Conditional on CONFIG_RMW_INSNS 5: Conditional on CONFIG_MN10300_HAS_ATOMIC_OPS_UNIT 6: SMP not supported? Uses generic cmpxchg. 7: Should use generic cmpxchg/cmpxchg_local? Setting __HAVE_ARCH_CMPXCHG a bug? > > +#define __HAVE_ARCH_CMPXCHG 1 > > Is it even possible to have CONFIG_SMP=y on an architecture that doesn't > support native cmpxchg? Certainly, the asm-generic cmpxchg.h barfs if > SMP, so couldn't we just change/extend the __HAVE_ARCH_CMPXCHG check in > rtmutex.c to work automatically with SMP architectures? That would help > arm64 too. Looks like there are a few arch-specific conditionals that might need to be worked out for arc, m68k, and mn10300; but this could potentially work. If we don't go down that route, we should probably at least add __HAVE_ARCH_CMPXCHG for arm64, blackfin (on SMP), frv, mips, tile, and xtensa as well. Josh -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
