linux-kernel - Re: [RANDOM] Move two variables to read

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Date:	Sun, 16 Dec 2007 18:33:12 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RANDOM] Move two variables to read_mostly section to save	memory

On Sun, Dec 16, 2007 at 07:38:14PM +0100, Eric Dumazet wrote:
> Adrian Bunk a ??crit :
> >On Sun, Dec 16, 2007 at 06:42:57PM +0100, Eric Dumazet wrote:
> >>Adrian Bunk a ??crit :
> >>...
> >>>And even more funny, with gcc 4.2 and CONFIG_CC_OPTIMIZE_FOR_SIZE=y your 
> >>>patch doesn't seem to make any space difference - are you using an older 
> >>>compiler or even worse CONFIG_CC_OPTIMIZE_FOR_SIZE=n for being able to 
> >>>see any space difference?
> >>>
> >>>In both cases your code uglification would be even more pointless...
> >>>
> >>I believe that CONFIG_SMP is uglification for you Adrian, but still I am 
> >>glad linux have it.
> >>
> >>If your CONFIG_SYSCTL=n is really that important for you, why dont you 
> >>define a new qualifier that can indeed mark some variables as :
> >>
> >>const if CONFIG_SYSCTL=n
> >>read_mostly if CONFIG_SYCTL=y
> >>
> >>This way you can keep compiler optimizations for your CONFIG_SYCTL=n, 
> >>while many people like me can still continue to optimize their kernel.
> >>
> >>Seeing so many sysctl already read_mostly in kernel, I wonder why you 
> >>NACK my patch, while it's easy to share your concerns with other people 
> >>and find a solution.
> >
> >You omitted an answer to my main important point...
> >
> >Let me ask it in a more simple way:
> >
> >Do you see any space difference at all with gcc 4.2 and 
> >CONFIG_CC_OPTIMIZE_FOR_SIZE=y ?
> >
> 
> 
> I am using gcc-4.2.1
> 
> CONFIG_CC_OPTIMIZE_FOR_SIZE=y makes no difference for me.
> 
> $ make defconfig
> $ egrep "OPTIMIZE_FOR_SIZE|CONFIG_SMP" .config
> CONFIG_CC_OPTIMIZE_FOR_SIZE=y
> CONFIG_SMP=y
> $ make vmlinux
> $ nm -v vmlinux|grep -4 random_read_wakeup_thresh
> c057a02c d excluded_id_list
> c057a100 d zero_bdi
> c057a180 D random_table
> c057a300 d input_pool
> c057a400 d random_read_wakeup_thresh
> c057a404 d random_write_wakeup_thresh
>    <SAME HOLE HERE>
> c057a480 d blocking_pool
> c057a580 d nonblocking_pool
> c057a680 d rekey_work
> 
> After my patch, I still gain 120 bytes.

Well there's really no point arguing about this. We've found the cause
of the hole (good), but moving other things around to magically fix it
is the wrong thing to do.

I'll queue a patch to remove the big ugly alignment.

Automatically detecting these sorts of holes in the kernel image would
be a useful thing to do. In a couple instances, I've spotted much
larger ones.

-- 
Mathematics is the supreme nostalgia of our time.
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