On Tue, 2008-09-30 at 12:37 -0500, Matt Mackall wrote:

quoted text
> 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

> #include

>

> 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.

Ok, on closer inspection, this is part of the x86_64 calling convention.

When calling a varargs function, the caller passes the number of

floating point SSE regs used in rax. The callee then has to save these

away for va_list use. The GCC prologue apparently sets aside space for

xmm0-xmm7 (16 bytes each) all the time (plus rdi, rsi, rdx, rcx, r8, and

r9).
Obviously, we're never passing floating point args in the kernel, so

we're taking about a 40+ byte hit in code size and 128 byte hit in stack

size for every varargs call.
Looks like the gcc people have a patch in progress:
http://gcc.gnu.org/ml/gcc-patches/2008-08/msg02165.html
So I think we should assume that x86_64 will sort this out eventually.
--

Mathematics is the supreme nostalgia of our time.
--

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