linux-kernel - Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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Message-Id: <1231368870.6686.16.camel@norville.austin.ibm.com>
Date:	Wed, 07 Jan 2009 16:54:30 -0600
From:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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

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