linux-kernel - Re: [PATCH RFC 1/3] mutex: Make more scalable by doing less atomic operations

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Message-ID: <20130410171654.GD21951@gmail.com>
Date:	Wed, 10 Apr 2013 19:16:54 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Clark Williams <williams@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Norton, Scott J" <scott.norton@...com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH RFC 1/3] mutex: Make more scalable by doing less atomic
 operations


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

> On 04/10/2013 06:31 AM, Ingo Molnar wrote:
> >* Waiman Long<Waiman.Long@...com>  wrote:
> >
> >>>That said, the MUTEX_SHOULD_XCHG_COUNT macro should die. Why shouldn't all
> >>>architectures just consider negative counts to be locked? It doesn't matter
> >>>that some might only ever see -1.
> >>I think so too. However, I don't have the machines to test out other
> >>architectures. The MUTEX_SHOULD_XCHG_COUNT is just a safety measure to make sure
> >>that my code won't screw up the kernel in other architectures. Once it is
> >>confirmed that a negative count other than -1 is fine for all the other
> >>architectures, the macro can certainly go.
> >I'd suggest to just remove it in an additional patch, Cc:-ing
> >linux-arch@...r.kernel.org. The change is very likely to be fine, if not then it's
> >easy to revert it.
> >
> >Thanks,
> >
> >	Ingo
>
> Yes, I can do that. So can I put your name down as reviewer or ack'er for the 
> 1st patch?

Since I'll typically the maintainer applying & pushing kernel/mutex.c changes to 
Linus via the locking tree, the commit will get a Signed-off-by from me once you 
resend the latest state of things - no need to add my Acked-by or Reviewed-by 
right now.

I'm still hoping for another patch from you that adds queueing to the spinners ... 
That approach could offer better performance than current patches 1,2,3. In 
theory.

I'd prefer that approach because you have a testcase that shows the problem and 
you are willing to maximize performance with it - so we could make sure we have 
reached maximum performance instead of dropping patches #2, #3, reaching partial 
performance with patch #1, without having a real full resolution.

Thanks,

	Ingo
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