linux-kernel - Re: [PATCH 3/3] block: convert elevator to generic rb tree code

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Date:	Tue, 29 May 2012 02:57:58 -0400
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	axboe@...nel.dk, paul.gortmaker@...driver.com
Subject: Re: [PATCH 3/3] block: convert elevator to generic rb tree code

On Tue, May 29, 2012 at 02:24:58PM +0900, Tejun Heo wrote:
> > So in practice there's no extra stack usage. Whether this is an
> > optimization we want to depend I'm not going to say; I suspect it's
> > pretty safe w.r.t. the optimizer but it's definitely sketchy and if at
> > some point someone came along and switched it to the uninline version
> > we'd have problems.
> 
> I don't think we can depend on that.  Note that compiler may as well
> decide not to inline an inline function (e.g. if it sees many calling
> instances).  Depending on such behavior is way too fragile.

Bah, I forgot about the compiler uninlining stuff. There's
__always_inline, but... yeah, I agree, too dangerous.

> > So we might want to leave this one open coded. Which would make me sad,
> > but I can't think of a sane way of implementing generic rb_search() that
> > doesn't require passing it a type t to compare against.
> 
> I don't know either.  Open coding isn't the end of the world but I
> suspect a lot of data structures which go on rbtree wouldn't be stack
> friendly, so having common helper which can't handle that might not be
> too helpful.

There's > 100 users in the kernel, I have no clue what the average size
of the containing struct is.

I think I'm gonna split rb_search() out into its own patch, as
rb_insert() fortunately doesn't have this problem.

I'm starting to think the sanest solution is a macro (not quite my
original RB_SEARCH() macro, though).
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