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

Ingo Molnar
2019-02-11T07:11:59Z

* Waiman Long <longman@redhat.com> wrote:

> On 02/10/2019 09:00 PM, 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 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