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

Linus Torvalds
2008-11-17T21:19:24Z

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