linux-kernel - Re: Large stack usage in fs code (especially for PPC64)

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Message-ID: <18722.5316.582974.95373@cargo.ozlabs.ibm.com>
Date:	Tue, 18 Nov 2008 12:05:08 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...abs.org, Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Large stack usage in fs code (especially for PPC64)

Steve,

> On Mon, 17 Nov 2008, Linus Torvalds wrote:
> > 
> > I do wonder just _what_ it is that causes the stack frames to be so 
> > horrid. For example, you have
> > 
> > 	 18)     8896     160   .kmem_cache_alloc+0xfc/0x140
> > 
> > and I'm looking at my x86-64 compile, and it has a stack frame of just 8 
> > bytes (!) for local variables plus the save/restore area (which looks like 
> > three registers plus frame pointer plus return address). IOW, if I'm 
> > looking at the code right (so big caveat: I did _not_ do a real stack 
> > dump!) the x86-64 stack cost for that same function is on the order of 48 
> > bytes. Not 160.
> 
> Out of curiosity, I just ran stack_trace on the latest version of git 
> (pulled sometime today) and ran it on my x86_64.
> 
> I have SLUB and SLUB debug defined, and here's what I found:
> 
>  11)     3592      64   kmem_cache_alloc+0x64/0xa3
> 
> 64 bytes, still much lower than the 160 of PPC64.

The ppc64 ABI has a minimum stack frame of 112 bytes, due to having an
area for called functions to store their parameters (64 bytes) plus 6
slots for saving stuff and for the compiler and linker to use if they
need to.  That's before any local variables are allocated.

The ppc32 ABI has a minimum stack frame of 16 bytes, which is much
nicer, at the expense of a much more complicated va_arg().

Paul.
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