linux-kernel - Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

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Date:	Tue, 01 Oct 2013 16:03:10 -0400
From:	Waiman Long <waiman.long@...com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

On 10/01/2013 03:33 AM, Ingo Molnar wrote:
> * Waiman Long<waiman.long@...com>  wrote:
>
>>> I think Waiman's patches (even the later ones) made the queued rwlocks
>>> be a side-by-side implementation with the old rwlocks, and I think
>>> that was just being unnecessarily careful. It might be useful for
>>> testing to have a config option to switch between the two, but we
>>> might as well go all the way.
>> It is not actually a side-by-side implementation. A user can choose
>> either regular rwlock or the queue one, but never both by setting a
>> configuration parameter. However, I now think that maybe we should do it
>> the lockref way by pre-determining it on a per-architecture level
>> without user visible configuration option.
> Well, as I pointed it out to you during review, such a Kconfig driven
> locking API choice is a no-go!
>
> What I suggested instead: there's absolutely no problem with providing a
> better rwlock_t implementation, backed with numbers and all that.
>
> Thanks,
>
> 	Ingo

Yes, this is what I am planning to do. The next version of my qrwlock 
patch will force the switch to queue rwlock for x86 architecture. The 
other architectures have to be done separately.

-Longman

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