Re: [PATCH v4 1/3] locking/rwsem: Remove arch specific rwsem files

Will Deacon
2019-02-20T14:47:17Z

On Wed, Feb 13, 2019 at 05:00:15PM -0500, Waiman Long wrote:
> As the generic rwsem-xadd code is using the appropriate acquire and
> release versions of the atomic operations, the arch specific rwsem.h
> files will not be that much faster than the generic code as long as the
> atomic functions are properly implemented. So we can remove those arch
> specific rwsem.h and stop building asm/rwsem.h to reduce maintenance
> effort.
>
> Currently, only x86, alpha and ia64 have implemented architecture
> specific fast paths. I don't have access to alpha and ia64 systems for
> testing, but they are legacy systems that are not likely to be updated
> to the latest kernel anyway.
>
> By using a rwsem microbenchmark, the total locking rates on a 4-socket
> 56-core 112-thread x86-64 system before and after the patch were as
> follows (mixed means equal # of read and write locks):
>
> Before Patch After Patch
> # of Threads wlock rlock mixed wlock rlock mixed
> ------------ ----- ----- ----- ----- ----- -----
> 1 29,201 30,143 29,458 28,615 30,172 29,201
> 2 6,807 13,299 1,171 7,725 15,025 1,804
> 4 6,504 12,755 1,520 7,127 14,286 1,345
> 8 6,762 13,412 764 6,826 13,652 726
> 16 6,693 15,408 662 6,599 15,938 626
> 32 6,145 15,286 496 5,549 15,487 511
> 64 5,812 15,495 60 5,858 15,572 60
>
> There were some run-to-run variations for the multi-thread tests. For
> x86-64, using the generic C code fast path seems to be a little bit
> faster than the assembly version with low lock contention. Looking at
> the assembly version of the fast paths, there are assembly to/from C
> code wrappers that save and restore all the callee-clobbered registers
> (7 registers on x86-64). The assembly generated from the generic C
> code doesn't need to do that. That may explain the slight performance
> gain here.
>
> The generic asm rwsem.h can also be merged into kernel/locking/rwsem.h
> with no code change as no other code other than those under
> kernel/locking needs to access the internal rwsem macros and functions.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> MAINTAINERS | 1 -
> arch/alpha/include/asm/rwsem.h | 211 -----------------------------------
> arch/arm/include/asm/Kbuild | 1 -
> arch/arm64/include/asm/Kbuild | 1 -
> arch/hexagon/include/asm/Kbuild | 1 -
> arch/ia64/include/asm/rwsem.h | 172 -----------------------------
> arch/powerpc/include/asm/Kbuild | 1 -
> arch/s390/include/asm/Kbuild | 1 -
> arch/sh/include/asm/Kbuild | 1 -
> arch/sparc/include/asm/Kbuild | 1 -
> arch/x86/include/asm/rwsem.h | 237 ----------------------------------------
> arch/x86/lib/Makefile | 1 -
> arch/x86/lib/rwsem.S | 156 --------------------------
> arch/x86/um/Makefile | 1 -
> arch/xtensa/include/asm/Kbuild | 1 -
> include/asm-generic/rwsem.h | 140 ------------------------
> include/linux/rwsem.h | 4 +-
> kernel/locking/percpu-rwsem.c | 2 +
> kernel/locking/rwsem.h | 130 ++++++++++++++++++++++
> 19 files changed, 133 insertions(+), 930 deletions(-)
> delete mode 100644 arch/alpha/include/asm/rwsem.h
> delete mode 100644 arch/ia64/include/asm/rwsem.h
> delete mode 100644 arch/x86/include/asm/rwsem.h
> delete mode 100644 arch/x86/lib/rwsem.S
> delete mode 100644 include/asm-generic/rwsem.h

Looks like a nice cleanup, thanks:

Acked-by: Will Deacon <will.deacon@arm.com>

Will