Re: [PATCH 18/22] block: Generic bio chaining

[Kent Overstreet]

On Wed, Aug 07, 2013 at 02:54:27PM -0700, Kent Overstreet wrote:
> This adds a generic mechanism for chaining bio completions. This is
> going to be used for a bio_split() replacement, and some other things in
> the future.
> 
> This is implemented with a new bio flag that bio_endio() checks; it
> would definitely be cleaner to implement chaining with a bi_end_io
> function, but since there's no limits on the depth of a bio chain (and
> with arbitrary bio splitting coming this is going to be a real issue)
> using an endio function would lead to unbounded stack usage.
> 
> Tail call optimization could solve that, but CONFIG_FRAME_POINTER
> disables gcc's tail call optimization (-fno-optimize-sibling-calls) - so
> we do it the hacky but safe way.

Btw, if you saw this patch and went "Wtf? What's the justification for
inflating struct bio and sticking another atomic op in the fast path?" -
here's the justification: The below patch gets me a 5% increase in
throughput (doing 4k random reads, and on one core on an old gulftown so
cpu bound).

(it also considerably simplifies a lot of random code, but there's a
real performance win to drivers handling arbitrary size bios so upper
layers don't have to care).