linux-kernel - Re: [PATCH 7/8] mm: use vm_unmapped

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Message-ID: <CANN689EJV_7Q7J4j1ttDxZuqbwD53PAuCHb5DhiE-AVbmNSR7Q@mail.gmail.com>
Date:	Tue, 8 Jan 2013 18:38:56 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Rik van Riel <riel@...hat.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Matt Turner <mattst88@...il.com>,
	David Howells <dhowells@...hat.com>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-parisc@...r.kernel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org
Subject: Re: [PATCH 7/8] mm: use vm_unmapped_area() on powerpc architecture

On Tue, Jan 8, 2013 at 6:15 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Tue, 2013-01-08 at 17:28 -0800, Michel Lespinasse wrote:
>> Update the powerpc slice_get_unmapped_area function to make use of
>> vm_unmapped_area() instead of implementing a brute force search.
>>
>> Signed-off-by: Michel Lespinasse <walken@...gle.com>
>>
>> ---
>>  arch/powerpc/mm/slice.c |  128 +++++++++++++++++++++++++++++-----------------
>>  1 files changed, 81 insertions(+), 47 deletions(-)
>
> That doesn't look good ... the resulting code is longer than the
> original, which makes me wonder how it is an improvement...

Well no fair, the previous patch (for powerpc as well) has 22
insertions and 93 deletions :)

The benefit is that the new code has lower algorithmic complexity, it
replaces a per-vma loop with O(N) complexity with an outer loop that
finds contiguous slice blocks and passes them to vm_unmapped_area()
which is only O(log N) complexity. So the new code will be faster for
workloads which use lots of vmas.

That said, I do agree that the code that looks for contiguous
available slices looks kinda ugly - just not sure how to make it look
nicer though.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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