linux-kernel - Re: [PATCH] slub: reduce total stack usage of slab_err & object

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Date:	Tue, 30 Sep 2008 17:20:38 +0100
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	penberg <penberg@...helsinki.fi>, mpm <mpm@...enic.com>,
	linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] slub: reduce total stack usage of slab_err & object_err

On Tue, 2008-09-30 at 10:38 -0500, Christoph Lameter wrote:
> Richard Kennedy wrote:
> > reduce the total stack usage of slab_err & object_err.
> > 
> > Introduce a new function to display a simple slab bug message, and call
> > this when vprintk is not needed.
> 
> You could simply get rid of the 100 byte buffer by using vprintk? Same method
> could be used elsewhere in the kernel and does not require additional
> functions. Compiles, untestted.
> 
> 
> 
> 
> Subject: Slub reduce slab_bug stack usage by using vprintk
> 
> Signed-off-by: Christoph Lameter <cl@...ux-foundation.org>
> 
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c	2008-09-30 10:34:40.000000000 -0500
> +++ linux-2.6/mm/slub.c	2008-09-30 10:36:10.000000000 -0500
> @@ -422,15 +422,14 @@
>  static void slab_bug(struct kmem_cache *s, char *fmt, ...)
>  {
>  	va_list args;
> -	char buf[100];
> 
>  	va_start(args, fmt);
> -	vsnprintf(buf, sizeof(buf), fmt, args);
> -	va_end(args);
>  	printk(KERN_ERR "========================================"
>  			"=====================================\n");
> -	printk(KERN_ERR "BUG %s: %s\n", s->name, buf);
> -	printk(KERN_ERR "----------------------------------------"
> +	printk(KERN_ERR "BUG %s: ", s->name);
> +	vprintk(fmt, args);
> +	va_end(args);
> +	printk(KERN_ERR "\n----------------------------------------"
>  			"-------------------------------------\n\n");
>  }
> 
Yes, using vprintk is better but you still have this path :
( with your patch applied)

	object_err -> slab_bug(208) -> printk(216)
instead of 
	object_err -> slab_bug_message(8) -> printk(216)

unfortunately the overhead for having var_args is pretty big, at least
on x86_64. I haven't measured it on 32 bit yet.

Richard


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