* Waiman Long [off-list ref] wrote:

On 02/10/2019 09:00 PM, Waiman Long wrote:

quoted

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        27,373  29,409  28,170    28,773  30,164  29,276
        2         7,697  14,922   1,703     7,435  15,167   1,729
        4         6,987  14,285   1,490     7,181  14,438   1,330
        8         6,650  13,652     761     6,918  13,796     718
       16         6,434  15,729     713     6,554  16,030     625
       32         5,590  15,312     552     6,124  15,344     471
       64         5,980  15,478      61     5,668  15,509      58

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 liitle bit
faster than the assembly version especially for read-lock and when lock
contention is low.  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
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>

I have decided to break the rwsem patchset that I sent out on last
Thursday into 3 parts. This patch is part 0 as it touches a number of
arch specific files and so have the widest distribution. I would like to
get it merged first. Part 1 will be patches 1-10 (except 4) of my
original rwsem patchset. This part moves things around, adds more
debugging capability and lays the ground work for the next part. Part 2
will contains the remaining patches which are the real beef of the whole
patchset.

Sounds good to me - I've merged this patch, will push it out after 
testing.

Thanks,

	Ingo