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

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Date:	Tue, 11 May 2010 15:07:21 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Mike Frysinger <vapier.adi@...il.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, 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.

-- 
Mathematics is the supreme nostalgia of our time.


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