linux-kernel - Re: Btrfs for mainline

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Message-ID: <20090104184103.GE2002@parisc-linux.org>
Date:	Sun, 4 Jan 2009 11:41:03 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: Btrfs for mainline

On Sun, Jan 04, 2009 at 07:21:50PM +0100, Peter Zijlstra wrote:
> The -rt tree has adaptive spin patches for the rtmutex code, its really
> not all that hard to do -- the rtmutex code is way more tricky than the
> regular mutexes due to all the PI fluff.
> 
> For kernel only locking the simple rule: spin iff the lock holder is
> running proved to be simple enough. Any added heuristics like max spin
> count etc. only made things worse. The whole idea though did make sense
> and certainly improved performance.

That implies moving

        struct thread_info      *owner;

out from under the CONFIG_DEBUG_MUTEXES code.  One of the original
justifications for mutexes was:

 - 'struct mutex' is smaller on most architectures: .e.g on x86,
   'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
    A smaller structure size means less RAM footprint, and better
    CPU-cache utilization.

I'd be reluctant to reverse that decision just for btrfs.

Benchmarking required!  Maybe I can put a patch together that implements
the simple 'spin if it's running' heuristic and throw it at our
testing guys on Monday ...

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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