linux-kernel - Re: [BUG] long freezes on thinkpad t60

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Date:	Thu, 21 Jun 2007 21:56:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Jarek Poplawski <jarkao2@...pl>,
	Miklos Szeredi <miklos@...redi.hu>, chris@...ee.ca,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: [BUG] long freezes on thinkpad t60

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Umm. i386 spinlocks could and should be *one*byte*.
> 
> In fact, I don't even know why they are wasting four bytes right now: 
> the fact that somebody made them an "int" just wastes memory. All the 
> actual code uses "decb", so it's not even a question of safety. I 
> wonder why we have that 32-bit thing and the ugly casts.
> 
> Ingo, any memory of that?

no real reason that i can recall - i guess nobody dared to touch it 
because it used to have that 'volatile', indicating black voodoo ;-) Now 
that the bad stigma has been removed, we could try the patch below. It 
boots fine here, and we save 1K of kernel text size:

     text    data     bss     dec     hex filename
  6236003  611992  401408 7249403  6e9dfb vmlinux.before
  6235075  611992  401408 7248475  6e9a5b vmlinux.after

I can understand why no data is saved by this change: gcc is aligning 
the next field to a natural boundary anyway and we dont really have 
arrays of spinlocks (fortunately). [and we save no data even if using 
the ((packed)) attribute.] Perhaps some data structure that is never in 
the kernel image itself still got smaller? Any good way to determine 
that?

But why is the text size different? Ah: i think it's spin_lock_init() 
getting shorter :-)

but this is certainly not something for 2.6.22, it's an early 2.6.23 
matter i suspect.

	Ingo

------------------->
From: Ingo Molnar <mingo@...e.hu>
Subject: [patch] spinlocks i386: change them to byte fields

all spinlock ops are on byte operands, so change the spinlock field to 
be unsigned char. This saves a bit of kernel text size:

   text    data     bss     dec     hex filename
6236003  611992  401408 7249403  6e9dfb vmlinux.before
6235075  611992  401408 7248475  6e9a5b vmlinux.after

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 include/asm-i386/spinlock_types.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux/include/asm-i386/spinlock_types.h
===================================================================
--- linux.orig/include/asm-i386/spinlock_types.h
+++ linux/include/asm-i386/spinlock_types.h
@@ -6,7 +6,7 @@
 #endif

 typedef struct {
-	unsigned int slock;
+	unsigned char slock;
 } raw_spinlock_t;

 #define __RAW_SPIN_LOCK_UNLOCKED	{ 1 }
-
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