linux-kernel - Re: [PATCH -mm v2 01/11] mm: track free size between VMAs in VMA rbtree

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Date:	Tue, 26 Jun 2012 10:31:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Rik van Riel <riel@...riel.com>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, aarcange@...hat.com, minchan@...il.com,
	kosaki.motohiro@...il.com, andi@...stfloor.org, hannes@...xchg.org,
	mel@....ul.ie, linux-kernel@...r.kernel.org, danielfsantos@....net
Subject: Re: [PATCH -mm v2 01/11] mm: track free size between VMAs in VMA
 rbtree

On Mon, 2012-06-25 at 17:52 -0400, Rik van Riel wrote:
> The downside? This makes the rbtree code somewhat more
> complex, vs. the brute force walk up the tree the current
> augmented rbtree code does. 

Something like that should be in the git history of that code. See
b945d6b2554d55 ("rbtree: Undo augmented trees performance damage and
regression").

I removed that because it adds overhead to the scheduler fast paths, but
if we can all agree to move lib/rbtree.c into inlines in
include/linux/rbtree.h (possibly utilizing Daniel Santos' magic) then we
could do this again.

Anyway, doing the updates in the insertion/deletion might speed up
those, but you still have the regular modifications what don't do
insert/delete to think about.

If you look at your patch 1, __vma_unlink has an adjust_free_gap() right
next to the rb_augment_erase(), vma_adjust() has 3 adjust_free_gap()
calls right next to each other.

All these will do an entire path walk back to the root. I would think we
could save quite a bit of updating by not having them all walk back to
the root. No point in re-computing the top levels if you know the next
update will change them again anyway.
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