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From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, linux-sh@vger.kernel.org,
	sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	linux-arch@vger.kernel.org, x86@kernel.org,
	Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH] locking/rwsem: Remove arch specific rwsem files
Date: Sun, 10 Feb 2019 21:08:40 -0500
Message-ID: <a610aeb2-b1cb-0d57-8078-97d1051481f9@redhat.com> (raw)
In-Reply-To: <1549850450-10171-1-git-send-email-longman@redhat.com>

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.

Cheers,
Longman


  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-11  2:00 Waiman Long
2019-02-11  2:08 ` Waiman Long [this message]
2019-02-11  7:11   ` Ingo Molnar
2019-02-11 10:39     ` Ingo Molnar
2019-02-11 10:52       ` Will Deacon
2019-02-11 10:55         ` Ingo Molnar
2019-02-11 13:32       ` Waiman Long
2019-02-11  9:36 ` Peter Zijlstra
2019-02-11  9:40   ` Peter Zijlstra
2019-02-11 10:57     ` Peter Zijlstra
2019-02-11 11:58 ` Peter Zijlstra
2019-02-11 16:35   ` Waiman Long
2019-02-11 17:04     ` Peter Zijlstra

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