From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning
Date: Wed, 07 Jan 2009 16:54:30 -0600
Message-ID: <1231368870.6686.16.camel@norville.austin.ibm.com>
References: <87r63ljzox.fsf@basil.nowhere.org>
	 <20090103191706.GA2002@parisc-linux.org> <1231242031.11687.97.camel@twins>
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	 <20090106131643.GA15228@elte.hu> <1231248041.11687.107.camel@twins>
	 <49636799.1010109@novell.com>  <20090106214229.GD6741@linux.vnet.ibm.com>
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	 <alpine.DEB.1.10.0901071530490.23456@gandalf.stny.rr.com>
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Cc: Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	paulmck@linux.vnet.ibm.com, Gregory Haskins <ghaskins@novell.com>,
	Ingo Molnar <mingo@elte.hu>, Matthew Wilcox <matthew@wil.cx>,
	Andi Kleen <andi@firstfloor.org>,
	Chris Mason <chris.mason@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nick Piggin <npiggin@suse.de>,
	Peter Morreale <pmorreale@novell.com>,
	Sven Dietrich <SDietrich@novell.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
In-Reply-To: <alpine.LFD.2.00.0901071355350.3057@localhost.localdomain>
List-ID: <linux-btrfs.vger.kernel.org>

On Wed, 2009-01-07 at 13:58 -0800, Linus Torvalds wrote:
> 
> On Wed, 7 Jan 2009, Peter Zijlstra wrote:
> > 
> > Do we really have to re-do all that code every loop?
> 
> No, you're right, we can just look up the cpu once. Which makes Andrew's 
> argument that "probe_kernel_address()" isn't in any hot path even more 
> true.

Do you need to even do that if CONFIG_DEBUG_PAGEALLOC is unset?
Something like:

#ifdef CONFIG_DEBUG_PAGEALLOC
                /*
                 * Need to access the cpu field knowing that
                 * DEBUG_PAGEALLOC could have unmapped it if
                 * the mutex owner just released it and exited.
                 */
                if (probe_kernel_address(&thread->cpu, cpu))
                        break;
#else
		cpu = thread->cpu;
#endif

Shaggy
-- 
David Kleikamp
IBM Linux Technology Center