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@xxxxxxxxxx>
---
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@xxxxxxx>
Will
