netdev - Re: [rfc][patch 1/3] slub: fix small HWCACHE

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Date:	Mon, 3 Mar 2008 13:28:57 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	yanmin_zhang@...ux.intel.com, David Miller <davem@...emloft.net>,
	Christoph Lameter <clameter@...r.sgi.com>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: [rfc][patch 1/3] slub: fix small HWCACHE_ALIGN alignment

On Mon, Mar 03, 2008 at 11:44:07AM +0200, Pekka Enberg wrote:
> Hi Nick,
> 
> On Mon, Mar 3, 2008 at 11:34 AM, Nick Piggin <npiggin@...e.de> wrote:
> >  SLUB should pack even small objects nicely into cachelines if that is what
> >  has been asked for. Use the same algorithm as SLAB for this.
> 
> The patches look good but this changelog is missing context. Why do we
> want to do this?

You're right, the changelog should be improved if/when it is merged.
Actually the context is discussed in another thread just now on lkml
if you are interested ("alloc_percpu() fails to allocate percpu data").

> 
> On Mon, Mar 3, 2008 at 11:34 AM, Nick Piggin <npiggin@...e.de> wrote:
> >  @@ -1896,12 +1896,15 @@ static unsigned long calculate_alignment
> >          * specified alignment though. If that is greater
> >          * then use it.
> >          */
> 
> You might want to fix the above comment too.

Yes.

> 
> >  -       if ((flags & SLAB_HWCACHE_ALIGN) &&
> >  -                       size > cache_line_size() / 2)
> >  -               return max_t(unsigned long, align, cache_line_size());
> >  +       if (flags & SLAB_HWCACHE_ALIGN) {
> >  +               unsigned long ralign = cache_line_size();
> >  +               while (size <= ralign / 2)
> >  +                       ralign /= 2;
> >  +               align = max(align, ralign);
> >  +       }
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