[PATCH] net: make neigh_priv_len in struct net_device 16bit instead of 8bit

[PATCH] net: make neigh_priv_len in struct net_device 16bit instead of 8bit

[Sebastian Andrzej Siewior]

neigh_priv_len is defined as u8. With all debug enabled struct
ipoib_neigh has 200 bytes. The largest part is sk_buff_head with 96
bytes and here the spinlock with 72 bytes.
The size value still fits in this u8 leaving some room for more.

On -RT struct ipoib_neigh put on weight and has 392 bytes. The main
reason is sk_buff_head with 288 and the fatty here is spinlock with 192
bytes. This does no longer fit into into neigh_priv_len and gcc
complains.

This patch changes neigh_priv_len from being 8bit to 16bit. Since the
following element (dev_id) is 16bit followed by a spinlock which is
aligned, the struct remains with a total size of 3200 (allmodconfig) /
2048 (with as much debug off as possible) bytes on x86-64.
On x86-32 the struct is 1856 (allmodconfig) / 1216 (with as much debug
off as possible) bytes long. The numbers were gained with and without
the patch to prove that this change does not increase the size of the
struct.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/netdevice.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 0ca8100..56da573 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1255,7 +1255,7 @@ struct net_device {
 	unsigned char		perm_addr[MAX_ADDR_LEN]; /* permanent hw address */
 	unsigned char		addr_assign_type; /* hw address assignment type */
 	unsigned char		addr_len;	/* hardware address length	*/
-	unsigned char		neigh_priv_len;
+	unsigned short		neigh_priv_len;
 	unsigned short          dev_id;		/* Used to differentiate devices
 						 * that share the same link
 						 * layer address
-- 
1.8.5.1

Re: [PATCH] net: make neigh_priv_len in struct net_device 16bit instead of 8bit

[Steven Rostedt]

On Thu, 12 Dec 2013 10:15:59 +0100
Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> neigh_priv_len is defined as u8. With all debug enabled struct
> ipoib_neigh has 200 bytes. The largest part is sk_buff_head with 96
> bytes and here the spinlock with 72 bytes.
> The size value still fits in this u8 leaving some room for more.
> 
> On -RT struct ipoib_neigh put on weight and has 392 bytes. The main
> reason is sk_buff_head with 288 and the fatty here is spinlock with 192
> bytes. This does no longer fit into into neigh_priv_len and gcc
> complains.
> 
> This patch changes neigh_priv_len from being 8bit to 16bit. Since the
> following element (dev_id) is 16bit followed by a spinlock which is
> aligned, the struct remains with a total size of 3200 (allmodconfig) /
> 2048 (with as much debug off as possible) bytes on x86-64.
> On x86-32 the struct is 1856 (allmodconfig) / 1216 (with as much debug
> off as possible) bytes long. The numbers were gained with and without
> the patch to prove that this change does not increase the size of the
> struct.
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  include/linux/netdevice.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 0ca8100..56da573 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1255,7 +1255,7 @@ struct net_device {
>  	unsigned char		perm_addr[MAX_ADDR_LEN]; /* permanent hw address */

MAX_ADDR_LEN is 32, thus it ends with proper alignment.

>  	unsigned char		addr_assign_type; /* hw address assignment type */
>  	unsigned char		addr_len;	/* hardware address length	*/
> -	unsigned char		neigh_priv_len;
> +	unsigned short		neigh_priv_len;

That means the new change should not affect the structure at all, as
the original structure has three chars followed by a short. This would
produce a 1 byte hole.

I wonder if Arnaldo's pahole tools would show this?

-- Steve

>  	unsigned short          dev_id;		/* Used to differentiate devices
>  						 * that share the same link
>  						 * layer address

Re: [PATCH] net: make neigh_priv_len in struct net_device 16bit instead of 8bit

[Arnaldo Carvalho de Melo]

Em Thu, Dec 12, 2013 at 10:57:20AM -0500, Steven Rostedt escreveu:
> On Thu, 12 Dec 2013 10:15:59 +0100
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> 
> > neigh_priv_len is defined as u8. With all debug enabled struct
> > ipoib_neigh has 200 bytes. The largest part is sk_buff_head with 96
> > bytes and here the spinlock with 72 bytes.
> > The size value still fits in this u8 leaving some room for more.
> > 
> > On -RT struct ipoib_neigh put on weight and has 392 bytes. The main
> > reason is sk_buff_head with 288 and the fatty here is spinlock with 192
> > bytes. This does no longer fit into into neigh_priv_len and gcc
> > complains.
> > 
> > This patch changes neigh_priv_len from being 8bit to 16bit. Since the
> > following element (dev_id) is 16bit followed by a spinlock which is
> > aligned, the struct remains with a total size of 3200 (allmodconfig) /
> > 2048 (with as much debug off as possible) bytes on x86-64.
> > On x86-32 the struct is 1856 (allmodconfig) / 1216 (with as much debug
> > off as possible) bytes long. The numbers were gained with and without
> > the patch to prove that this change does not increase the size of the
> > struct.
> > 
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  include/linux/netdevice.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 0ca8100..56da573 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -1255,7 +1255,7 @@ struct net_device {
> >  	unsigned char		perm_addr[MAX_ADDR_LEN]; /* permanent hw address */
> 
> MAX_ADDR_LEN is 32, thus it ends with proper alignment.
> 
> >  	unsigned char		addr_assign_type; /* hw address assignment type */
> >  	unsigned char		addr_len;	/* hardware address length	*/
> > -	unsigned char		neigh_priv_len;
> > +	unsigned short		neigh_priv_len;
> 
> That means the new change should not affect the structure at all, as
> the original structure has three chars followed by a short. This would
> produce a 1 byte hole.
> 
> I wonder if Arnaldo's pahole tools would show this?

Yes, it will, in a recent build here:

pahole -C net_device ../build/v3.13.0-rc1+/net/socket.o
<SNIP>
        unsigned char              perm_addr[32];        /*   532    32 */
        unsigned char              addr_assign_type;     /*   564     1 */
        unsigned char              addr_len;             /*   565     1 */
        unsigned char              neigh_priv_len;       /*   566     1 */

        /* XXX 1 byte hole, try to pack */

        short unsigned int         dev_id;               /*   568     2 */
<SNIP>

So, yeah, one can promote neigh_priv_len from 'unsigned char' to 'unsigned
short' and the result will have the same sizeof and layout, modulo that hole,
that will disappear.
 
> -- Steve
> 
> >  	unsigned short          dev_id;		/* Used to differentiate devices
> >  						 * that share the same link
> >  						 * layer address

Re: [PATCH] net: make neigh_priv_len in struct net_device 16bit instead of 8bit

[David Miller]

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu, 12 Dec 2013 10:15:59 +0100

> neigh_priv_len is defined as u8. With all debug enabled struct
> ipoib_neigh has 200 bytes. The largest part is sk_buff_head with 96
> bytes and here the spinlock with 72 bytes.
> The size value still fits in this u8 leaving some room for more.
> 
> On -RT struct ipoib_neigh put on weight and has 392 bytes. The main
> reason is sk_buff_head with 288 and the fatty here is spinlock with 192
> bytes. This does no longer fit into into neigh_priv_len and gcc
> complains.
> 
> This patch changes neigh_priv_len from being 8bit to 16bit. Since the
> following element (dev_id) is 16bit followed by a spinlock which is
> aligned, the struct remains with a total size of 3200 (allmodconfig) /
> 2048 (with as much debug off as possible) bytes on x86-64.
> On x86-32 the struct is 1856 (allmodconfig) / 1216 (with as much debug
> off as possible) bytes long. The numbers were gained with and without
> the patch to prove that this change does not increase the size of the
> struct.
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

This is fine, applied, thanks.