linux-kernel - Re: [PATCH v4 0/3] locking/rwsem: Rwsem rearchitecture part 0

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Date:   Mon, 18 Feb 2019 14:58:20 +0000
From:   Will Deacon <will.deacon@....com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-c6x-dev@...ux-c6x.org,
        uclinux-h8-devel@...ts.sourceforge.jp,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        nios2-dev@...ts.rocketboards.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v4 0/3] locking/rwsem: Rwsem rearchitecture part 0

On Fri, Feb 15, 2019 at 01:58:34PM -0500, Waiman Long wrote:
> On 02/15/2019 01:40 PM, Will Deacon wrote:
> > On Thu, Feb 14, 2019 at 11:37:15AM +0100, Peter Zijlstra wrote:
> >> On Wed, Feb 13, 2019 at 05:00:14PM -0500, Waiman Long wrote:
> >>> v4:
> >>>  - Remove rwsem-spinlock.c and make all archs use rwsem-xadd.c.
> >>>
> >>> v3:
> >>>  - Optimize __down_read_trylock() for the uncontended case as suggested
> >>>    by Linus.
> >>>
> >>> v2:
> >>>  - Add patch 2 to optimize __down_read_trylock() as suggested by PeterZ.
> >>>  - Update performance test data in patch 1.
> >>>
> >>> The goal of this patchset is to remove the architecture specific files
> >>> for rwsem-xadd to make it easer to add enhancements in the later rwsem
> >>> patches. It also removes the legacy rwsem-spinlock.c file and make all
> >>> the architectures use one single implementation of rwsem - rwsem-xadd.c.
> >>>
> >>> Waiman Long (3):
> >>>   locking/rwsem: Remove arch specific rwsem files
> >>>   locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all
> >>>     archs
> >>>   locking/rwsem: Optimize down_read_trylock()
> >> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> >>
> >> with the caveat that I'm happy to exchange patch 3 back to my earlier
> >> suggestion in case Will expesses concerns wrt the ARM64 performance of
> >> Linus' suggestion.
> > Right, the current proposal doesn't work well for us, unfortunately. Which
> > was your earlier suggestion?
> >
> > Will
> 
> In my posting yesterday, I showed that most of the trylocks done were
> actually uncontended. Assuming that pattern hold for the most of the
> workloads, it will not that bad after all.

That's fair enough; if you're going to sit in a tight trylock() loop like the
benchmark does, then you're much better off just calling lock() if you care
at all about scalability.

Will