linux-kernel - Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

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Message-ID: <AANLkTimt-eRCt_HTq7OUwzG1dgRuzJHk1tlgOcq4ocL7@mail.gmail.com>
Date:	Tue, 11 May 2010 16:10:31 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oskar Schirmer <os@...ix.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Daniel Glöckner <dg@...ix.com>,
	Oliver Schneidewind <osw@...ix.com>,
	Johannes Weiner <jw@...ix.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

On Tue, May 11, 2010 at 16:07, Matt Mackall wrote:
> On Tue, 2010-05-11 at 16:03 -0400, Mike Frysinger wrote:
>> On Tue, May 11, 2010 at 02:42, Pekka Enberg wrote:
>> > On Tue, May 11, 2010 at 9:33 AM, Dmitry Torokhov wrote:
>> >>> what guarantee exactly do you have for that statement ?
>> >>
>> >> The data is kmalloced, kmalloc aligns on cacheline boundary AFAIK which
>> >> means that next kmalloc data chunk will not share "our" cacheline.
>> >
>> > No, there are no such guarantees. kmalloc() aligns on
>> > ARCH_KMALLOC_MINALIGN or ARCH_SLAB_MINALIGN depending on which is
>> > bigger but beyond that, there are no guarantees. You can, of course,
>> > use kmem_cache_create() with SLAB_HWCACHE_ALIGN to align on cacheline
>> > boundary.
>>
>> so how is this to be addressed in general ?  this is a problem for any
>> device that does SPI transactions, and having every driver create its
>> own kmem cache isnt the answer.
>>
>> do people need to kmalloc() like 2x the desired size and manually
>> align it themselves ?  declaring alignments on struct members doesnt
>> matter if the base of the struct isnt aligned.  seems like we need a
>> new GFP flag that says we need a cache aligned pointer so we can give
>> that to kmalloc() and such.
>
> Make your own slab cache with the alignment flag set, as Pekka already
> mentioned.

and like i said, that doesnt sound like a reasonable solution when
every single SPI driver (over 100 atm) out there is affected
-mike
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