Re: [PATCH 0/4] Update definition of cmpxchg() under CodeSamples

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Wed, Dec 12, 2018 at 12:39:38AM +0900, Akira Yokosawa wrote:
> Subject: [PATCH 0/4] Update definition of cmpxchg() under CodeSamples
> 
> Hi Paul and Junchang,
> 
> Based on the earlier correspondence, I prepared a patch set to update
> the definition of cmpxchg().
> 
> Patch #1 adds a simple litmus test of cmpxchg().
> Patch #2 fix cmpxchg() in .../litmus/api.h by specifying strong
> __atomic_compare_exchange_n)\(). Results of litmus tests are presented
> in the commit log.
> Patch #3 fix cmpxchg() in .../api-gcc.h in the same manner as patch #2.

I applied and pushed #1-3, thank you!

							Thanx, Paul

> Patch #4 is not intended to be applied to master, but provides a weak
> variant of cmpxchg() and count_lim_atomic_weak.c.
> 
> I did some experiments on PPC (10 runs each).
> 
> count_lim_atomic 1 uperf, ns/update (as of v2018.12.08a):
> 56.6572
> 56.4972
> 56.5504
> 56.4972
> 56.6305
> 56.5238
> 56.6038
> 56.5238
> 56.5504
> 56.5238
> 
> count_lim_atomic 1 uperf, ns/update (as of Patch #4):
> 38.3264
> 38.1073
> 38.2287
> 38.2287
> 38.2653
> 38.3142
> 38.1194
> 38.1801
> 38.4986
> 38.2531
> 
> count_lim_atomic_weak 1 uperf, ns/update (as of Patch #4):
> 47.3186
> 47.3747
> 47.4121
> 47.5248
> 47.3934
> 47.3373
> 47.3186
> 47.4121
> 47.6002
> 47.3186
> 
> So count_lim_atomic (strong semantics) is around 9ns/update
> faster than count_lim_atomic_weak, and is around 18ns/update
> faster than count_lim_atomic of v2018.12.08a.
> 
> This contradicts Junchang's view. The difference looks like
> due to removal of ternary operation in cmpxchg().
> 
> Or I might be missing something.
> 
> Junchang, can you please experiment on PPC and ARM?
> 
>         Thanks, Akira
> --
> Akira Yokosawa (4):
>   CodeSamples: Add C-cmpxchg.litmus
>   CodeSamples/formal/litmus/api.h: Fix definition of cmpxchg()
>   CodeSamples: Fix definition of cmpxchg() in api-gcc.h
>   [EXP] CodeSamples: Add weak variant of cmpxchg() as cmpxchg_weak()
> 
>  CodeSamples/api-pthreads/api-gcc.h         |  19 ++-
>  CodeSamples/count/Makefile                 |   4 +
>  CodeSamples/count/count_lim_atomic_weak.c  | 226 +++++++++++++++++++++++++++++
>  CodeSamples/formal/litmus/C-cmpxchg.litmus |  24 +++
>  CodeSamples/formal/litmus/api.h            |   2 +-
>  5 files changed, 272 insertions(+), 3 deletions(-)
>  create mode 100644 CodeSamples/count/count_lim_atomic_weak.c
>  create mode 100644 CodeSamples/formal/litmus/C-cmpxchg.litmus
> 
> -- 
> 2.7.4
>