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

From: Will Deacon <will.deacon@arm.com>
To: Waiman Long <longman@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
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	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 v4 0/3] locking/rwsem: Rwsem rearchitecture part 0
Date: Mon, 18 Feb 2019 14:58:20 +0000	[thread overview]
Message-ID: <20190218145820.GA16091@fuggles.cambridge.arm.com> (raw)
In-Reply-To: <a45d8b68-5623-d6fe-8080-072994f7625e@redhat.com>

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@infradead.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