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

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Message-Id: <1222796245.23159.38.camel@calx>
Date:	Tue, 30 Sep 2008 12:37:25 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	penberg <penberg@...helsinki.fi>, linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] slub: reduce total stack usage of slab_err & object_err


On Tue, 2008-09-30 at 17:20 +0100, Richard Kennedy wrote:
> 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.

That's fascinating. I tried a simple test case in userspace:

#include <stdarg.h>
#include <stdio.h>

void p(char *fmt, ...)
{
	va_list args;

	va_start(args, fmt);
	vprintf(fmt, args);
	va_end(args);
}

On 32-bit, I'm seeing 32 bytes of stack vs 216 on 64-bit. Disassembly
suggests it's connected to va_list fiddling with XMM registers, which
seems quite odd.

-- 
Mathematics is the supreme nostalgia of our time.

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