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

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Date:	Mon, 17 Nov 2008 13:18:44 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.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)

On Mon, 17 Nov 2008, Steven Rostedt wrote:
> 
> Here's my stack after boot up with CONFIG_IRQSTACKS set. Seems that 
> softirqs still use the same stack as the process.

Yes.

> This is still 12K. Kind of big even for a 16K stack.

And while that 1kB+ stack slot for block_read_full_page still stands out 
like a sore thumb, I do agree that there's way too many other functions 
too with big stack frames.

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.

Where does that factor-of-three+ difference come from? From the numbers, I 
suspect ppc64 has a 32-byte stack alignment, which may be part of it, and 
I guess the compiler is more eager to use all those extra registers and 
will happily have many more callee-saved regs that are actually used.

But that still a _lot_ of extra stack.

Of course, you may have things like spinlock debugging etc enabled. Some 
of our debugging options do tend to blow things up.

			Linus
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