linux-kernel - Re: [PATCH 05/10] mm, util: Use dup

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Message-Id: <20120926144219.bf4bfb9d.akpm@linux-foundation.org>
Date:	Wed, 26 Sep 2012 14:42:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ezequiel Garcia <elezegarcia@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH 05/10] mm, util: Use dup_user to duplicate user memory

On Tue, 25 Sep 2012 22:15:38 -0300
Ezequiel Garcia <elezegarcia@...il.com> wrote:

> > This patch increases util.o's text size by 238 bytes.  A larger kernel
> > with a worsened cache footprint.
> >
> > And we did this to get marginally improved tracing output?  This sounds
> > like a bad tradeoff to me.
> >
> 
> Mmm, that's bad tradeoff indeed.
> It's certainly odd since the patch shouldn't increase the text size
> *that* much.
> Is it too much to ask that you send your kernel config and gcc version.

x86_64 allmodconfig with CONFIG_DEBUG_INFO=n,
CONFIG_ENABLE_MUST_CHECK=n. gcc-4.4.4.

> My compilation (x86 kernel in gcc 4.7.1) shows a kernel less bloated:
> 
> $ readelf -s util-dup-user.o | grep dup_user
>    161: 00001c10   108 FUNC    GLOBAL DEFAULT    1 memdup_user
>    169: 00001df0   159 FUNC    GLOBAL DEFAULT    1 strndup_user
> $ readelf -s util.o | grep dup_user
>    161: 00001c10   108 FUNC    GLOBAL DEFAULT    1 memdup_user
>    169: 00001df0    98 FUNC    GLOBAL DEFAULT    1 strndup_user
> 
> $ size util.o
>    text	   data	    bss	    dec	    hex	filename
>   18319	   2077	      0	  20396	   4fac	util.o
> $ size util-dup-user.o
>    text	   data	    bss	    dec	    hex	filename
>   18367	   2077	      0	  20444	   4fdc	util-dup-user.o
> 
> Am I doing anything wrong?

Dunno - it could be a config thing.

> If you still feel this is unnecessary bloatness, perhaps I could think of
> something depending on CONFIG_TRACING (though I know
> we all hate those nasty ifdefs).

hm.  Perhaps we could add an __always_inline_for_tracing.  But that
wouldn't help a lot - a CONFIG_TRACING_SUPPORT=y kernel would still be
impacted even if the user is never using tracing.

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