Re: [PATCH] slub: reduce total stack usage of slab_err & object_err

Matt Mackall
2008-09-30T17:40:53Z

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.