linux-kernel - Re: [PATCH 3/3] brlocks/lglocks: turn into functions

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Message-ID: <20120420112149.GH25458@amd.local0.net>
Date:	Fri, 20 Apr 2012 21:21:49 +1000
From:	Nick Piggin <npiggin@...nel.dk>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/3] brlocks/lglocks: turn into functions

This still not merged?

On Mon, Mar 05, 2012 at 05:35:28PM +1030, Rusty Russell wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> lglocks and brlocks are currently generated with some complicated macros
> in lglock.h.  But there's no reason to not just use common utility
> functions and put all the data into a common data structure.

There is a reason, which is performance. Extra function call, but also
IIRC the percpu accessor was not so fast doing it this way. Maybe
that's improved...

So what's the performance difference?

> 
> Since there are at least two users it makes sense to share this code in a
> library.  This is also easier maintainable than a macro forest.
> 
> This will also make it later possible to dynamically allocate lglocks and
> also use them in modules (this would both still need some additional, but
> now straightforward, code)

Yes, but let's not do either of those things :)

I was slightly crazy when committing that patch to the kernel, I'll
admit. So if performance isn't significantly affected, then definitely.
If it is... well, it's much easier to gain 1% performance by maintaining
100 self contained lines of hilarious code like this than to actually
use your brain to improve somewhere else!

Thanks,
Nick
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